…s Module-Friendly (#207)

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### 🕓 Changelog

We bump the Vyper version to the latest breaking release `0.4.0`
(technically, since Vyper `0.4.0` has not been released yet, we use the
latest release candidate
`0.4.0rc6` (https://github.com/vyperlang/vyper/releases/tag/v0.4.0rc6)
instead), which implements _stateless_ (see
vyperlang/vyper@c6f457a)
and _stateful_ modules (see
vyperlang/vyper@8ccacb3).
Furthermore, all existing `🐍 snekmate` contracts have been refactored to
be module-friendly (closes #221):

- [x] Authentication
  - [x] `ownable` (#218)
  - [x] `ownable_2step` (#219)
  - [x] `access_control` (#216)
- [x] Extensions
  - [x] `erc2981` (#233)
  - [x] `erc4626` (#236)
- [x] Governance
  - [x] `timelock_controller` (#220)
- [x] Tokens
  - [x] `erc20` (#234)
  - [x] `erc721` (#237)
  - [x] `erc1155` (#238)
- [x] Utility Functions
  - [x] `base64` (#222)
  - [x] `batch_distributor` (#223)
  - [x] `create_address` (#224)
  - [x] `create2_address` (#225)
  - [x] `ecdsa` (#227)
  - [x] `message_hash_utils` (#227)
  - [x] `signature_checker` (#228)
  - [x] `eip712_domain_separator` (#229)
  - [x] `math` (#230)
  - [x] `merkle_proof_verification` (#231)
  - [x] `multicall` (#232)

Furthermore, this PR closes the following issues:
- [x] Closes #175 (#243),
- [x] Closes #184 (#239),
- [x] Closes #180 (#240).

Eventually, the following changes are implemented:
- [x] Use `snake_case` notation for module and mock contracts. (#242)
- [x] The mathematical utility functions `_log_2`, `_log_10`, and
`_log_256` are renamed to `_log2`, `_log10`, and `_log256`. (#242)
- [x] Add documentation on how to use 🐍 snekmate contracts as modules.
(#247)

---------

Signed-off-by: Pascal Marco Caversaccio <[email protected]>