[CAP-0046-02] Updated contract lifecycle CAP.

The main updates:

- Change `InvokeHostFunctionOp` to use XDR for contract creation args, as it's getting quite complex and doesn't ever need to be called from the contracts.
- Decouple WASM sources from the contract instances in order to deduplicate contracts that share their code.
- Change the contract-from-contract creation to use the code reference instead of the actual code
- Removed some outdated parts that don't allow well with the current implementation/vision

contents/cap-0046/Stellar-contract.x

@@ -250,14 +250,14 @@ case NEGATIVE:
 
 enum SCContractCodeType
 {
-    SCCONTRACT_CODE_WASM = 0,
+    SCCONTRACT_CODE_WASM_REF = 0,
     SCCONTRACT_CODE_TOKEN = 1
 };
 
 union SCContractCode switch (SCContractCodeType type)
 {
-case SCCONTRACT_CODE_WASM:
-    opaque wasm<SCVAL_LIMIT>;
+case SCCONTRACT_CODE_WASM_REF:
+    Hash wasm_id;
 case SCCONTRACT_CODE_TOKEN:
     void;
 };