refactor: use immutable properties where possible (#5)

This commit changes a few token properties to be `immutable` as this
reduces gas costs even further.

This was reported by Slither.

A gas report with changes in gas cost is attached.

.gas-snapshot

@@ -2,11 +2,11 @@ AddEntryTest:test_AddEntry() (gas: 44392)
 AddEntryTest:test_RevertWhen_EntryAlreadyExists() (gas: 42644)
 AddEntryTest:test_RevertWhen_InvalidAddress() (gas: 25133)
 AddEntryTest:test_RevertWhen_SenderIsNotTokenDeployer() (gas: 14827)
-CollectibleV1Test:test_Deployment() (gas: 38626)
-CommunityERC20Test:test_Deployment() (gas: 29720)
+CollectibleV1Test:test_Deployment() (gas: 36386)
+CommunityERC20Test:test_Deployment() (gas: 27614)
 CommunityTokenDeployerTest:test_Deployment() (gas: 14805)
-CreateTest:test_Create() (gas: 2216454)
-CreateTest:test_Create() (gas: 2489668)
+CreateTest:test_Create() (gas: 2251262)
+CreateTest:test_Create() (gas: 2548166)
 CreateTest:test_RevertWhen_InvalidOwnerTokenAddress() (gas: 15523)
 CreateTest:test_RevertWhen_InvalidReceiverAddress() (gas: 15656)
 CreateTest:test_RevertWhen_InvalidSignerPublicKey() (gas: 17057)
@@ -16,33 +16,33 @@ CreateTest:test_RevertWhen_SenderIsNotTokenDeployer() (gas: 16421)
 CreateTest:test_RevertWhen_SenderIsNotTokenDeployer() (gas: 16524)
 DeployContracts:test() (gas: 120)
 DeployOwnerAndMasterToken:test() (gas: 120)
-DeployTest:test_Deploy() (gas: 4778776)
+DeployTest:test_Deploy() (gas: 4872082)
 DeployTest:test_Deployment() (gas: 14947)
-DeployTest:test_RevertWhen_AlreadyDeployed() (gas: 4774645)
+DeployTest:test_RevertWhen_AlreadyDeployed() (gas: 4868312)
 DeployTest:test_RevertWhen_InvalidCommunityAddress() (gas: 51385)
 DeployTest:test_RevertWhen_InvalidDeployerAddress() (gas: 55272)
 DeployTest:test_RevertWhen_InvalidDeploymentSignature() (gas: 65617)
 DeployTest:test_RevertWhen_InvalidSignerPublicKey() (gas: 53433)
-DeployTest:test_RevertWhen_InvalidTokenMetadata() (gas: 2613184)
+DeployTest:test_RevertWhen_InvalidTokenMetadata() (gas: 2671682)
 DeploymentTest:test_Deployment() (gas: 14671)
 DeploymentTest:test_Deployment() (gas: 14671)
 DeploymentTest:test_Deployment() (gas: 17295)
 GetEntryTest:test_ReturnZeroAddressIfEntryDoesNotExist() (gas: 11906)
-MintToTest:test_Deployment() (gas: 29742)
-MintToTest:test_Deployment() (gas: 38626)
-MintToTest:test_Deployment() (gas: 85460)
-MintToTest:test_MintTo() (gas: 509094)
+MintToTest:test_Deployment() (gas: 27636)
+MintToTest:test_Deployment() (gas: 36386)
+MintToTest:test_Deployment() (gas: 83220)
+MintToTest:test_MintTo() (gas: 506888)
 MintToTest:test_RevertWhen_AddressesAndAmountsAreNotEqualLength() (gas: 23778)
-MintToTest:test_RevertWhen_MaxSupplyIsReached() (gas: 22859)
-MintToTest:test_RevertWhen_MaxSupplyIsReached() (gas: 505067)
+MintToTest:test_RevertWhen_MaxSupplyIsReached() (gas: 20653)
+MintToTest:test_RevertWhen_MaxSupplyIsReached() (gas: 502655)
 MintToTest:test_RevertWhen_MaxSupplyReached() (gas: 122988)
-MintToTest:test_RevertWhen_SenderIsNotOwner() (gas: 35956)
-OwnerTokenTest:test_Deployment() (gas: 85460)
-RemoteBurnTest:test_Deployment() (gas: 38626)
-RemoteBurnTest:test_Deployment() (gas: 85482)
-RemoteBurnTest:test_RemoteBurn() (gas: 455309)
-RemoteBurnTest:test_RevertWhen_RemoteBurn() (gas: 19091)
-RemoteBurnTest:test_RevertWhen_SenderIsNotOwner() (gas: 24791)
+MintToTest:test_RevertWhen_SenderIsNotOwner() (gas: 31544)
+OwnerTokenTest:test_Deployment() (gas: 83220)
+RemoteBurnTest:test_Deployment() (gas: 36386)
+RemoteBurnTest:test_Deployment() (gas: 83242)
+RemoteBurnTest:test_RemoteBurn() (gas: 450780)
+RemoteBurnTest:test_RevertWhen_RemoteBurn() (gas: 14768)
+RemoteBurnTest:test_RevertWhen_SenderIsNotOwner() (gas: 20379)
 SetCommunityTokenDeployerAddressTest:test_RevertWhen_InvalidTokenDeployerAddress() (gas: 12941)
 SetCommunityTokenDeployerAddressTest:test_RevertWhen_SenderIsNotOwner() (gas: 12482)
 SetCommunityTokenDeployerAddressTest:test_SetCommunityTokenDeployerAddress() (gas: 22808)
@@ -54,20 +54,20 @@ SetMasterTokenFactoryAddressTest:test_Deployment() (gas: 14805)
 SetMasterTokenFactoryAddressTest:test_RevertWhen_InvalidTokenFactoryAddress() (gas: 12992)
 SetMasterTokenFactoryAddressTest:test_RevertWhen_SenderIsNotOwner() (gas: 12465)
 SetMasterTokenFactoryAddressTest:test_SetOwnerTokenFactoryAddress() (gas: 22861)
-SetMaxSupplyTest:test_Deployment() (gas: 29720)
-SetMaxSupplyTest:test_Deployment() (gas: 85482)
-SetMaxSupplyTest:test_RevertWhen_CalledBecauseMaxSupplyIsLocked() (gas: 16533)
+SetMaxSupplyTest:test_Deployment() (gas: 27614)
+SetMaxSupplyTest:test_Deployment() (gas: 83242)
+SetMaxSupplyTest:test_RevertWhen_CalledBecauseMaxSupplyIsLocked() (gas: 14327)
 SetMaxSupplyTest:test_RevertWhen_MaxSupplyLowerThanTotalSupply() (gas: 148572)
+SetMaxSupplyTest:test_RevertWhen_SenderIsNotOwner() (gas: 12527)
 SetMaxSupplyTest:test_RevertWhen_SenderIsNotOwner() (gas: 12817)
-SetMaxSupplyTest:test_RevertWhen_SenderIsNotOwner() (gas: 16939)
 SetMaxSupplyTest:test_SetMaxSupply() (gas: 15597)
 SetOwnerTokenFactoryAddressTest:test_Deployment() (gas: 14805)
 SetOwnerTokenFactoryAddressTest:test_RevertWhen_InvalidTokenFactoryAddress() (gas: 12970)
 SetOwnerTokenFactoryAddressTest:test_RevertWhen_SenderIsNotOwner() (gas: 12443)
 SetOwnerTokenFactoryAddressTest:test_SetOwnerTokenFactoryAddress() (gas: 22840)
-SetSignerPublicKeyTest:test_Deployment() (gas: 85460)
-SetSignerPublicKeyTest:test_RevertWhen_SenderIsNotOwner() (gas: 17634)
-SetSignerPublicKeyTest:test_SetSignerPublicKey() (gas: 26369)
+SetSignerPublicKeyTest:test_Deployment() (gas: 83220)
+SetSignerPublicKeyTest:test_RevertWhen_SenderIsNotOwner() (gas: 13222)
+SetSignerPublicKeyTest:test_SetSignerPublicKey() (gas: 24163)
 SetTokenDeployerAddressTest:test_RevertWhen_InvalidTokenDeployerAddress() (gas: 12964)
 SetTokenDeployerAddressTest:test_RevertWhen_InvalidTokenDeployerAddress() (gas: 12964)
 SetTokenDeployerAddressTest:test_RevertWhen_SenderIsNotOwner() (gas: 12438)

contracts/tokens/BaseToken.sol

@@ -27,18 +27,18 @@ abstract contract BaseToken is Context, ERC721Enumerable {
      */
     uint256 public maxSupply;
 
-    address public ownerToken;
-    address public masterToken;
+    address public immutable ownerToken;
+    address public immutable masterToken;
 
     /**
      * If set to true, the contract owner can burn any token.
      */
-    bool public remoteBurnable;
+    bool public immutable remoteBurnable;
 
     /**
      * If set to false it acts as a soulbound token.
      */
-    bool public transferable;
+    bool public immutable transferable;
 
     string public baseTokenURI;
 

contracts/tokens/CommunityERC20.sol

@@ -15,7 +15,7 @@ contract CommunityERC20 is Context, Ownable, ERC20 {
      */
     uint256 public maxSupply;
 
-    uint8 private customDecimals;
+    uint8 private immutable customDecimals;
 
     constructor(
         string memory _name,