♻️ Make ERC4626 Module-Friendly (#236)

### 🕓 Changelog

This PR refactors the `ERC4626` contract to make it module-friendly and
ready for the breaking `0.4.0` release. Please note that due to an open
issue (see vyperlang/vyper#3954) on how to
handle public getters of interface types in conjunction with
`implements`, we fix this issue by storing the interface object as an
`immutable` variable called `_ASSET` and assigning `asset` a `public`
`immutable` `address` type, which gets assigned at contract creation
time via the `.address` member of `_ASSET`:

```vy
asset: public(immutable(address))
_ASSET: immutable(IERC20)

@Deploy
@payable
def __init__(..., asset_: IERC20, ...):
    _ASSET = asset_
    asset = _ASSET.address

    ....
```

---------

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

CHANGELOG.md

@@ -10,6 +10,7 @@
   - [`AccessControl`](https://github.com/pcaversaccio/snekmate/blob/v0.1.0/src/snekmate/auth/AccessControl.vy): Make `AccessControl` module-friendly. ([#216](https://github.com/pcaversaccio/snekmate/pull/216))
 - **Extensions**
   - [`ERC2981`](https://github.com/pcaversaccio/snekmate/blob/v0.1.0/src/snekmate/extensions/ERC2981.vy): Make `ERC2981` module-friendly. ([#233](https://github.com/pcaversaccio/snekmate/pull/233))
+  - [`ERC4626`](https://github.com/pcaversaccio/snekmate/blob/v0.1.0/src/snekmate/extensions/ERC4626.vy): Make `ERC4626` module-friendly. ([#236](https://github.com/pcaversaccio/snekmate/pull/236))
 - **Governance**
   - [`TimelockController`](https://github.com/pcaversaccio/snekmate/blob/v0.1.0/src/snekmate/governance/TimelockController.vy): Make `TimelockController` module-friendly. ([#220](https://github.com/pcaversaccio/snekmate/pull/220))
 - **Tokens**

README.md

@@ -33,7 +33,8 @@ src
     │   ├── interfaces
     │   │   └── IERC2981 — "EIP-2981 Interface Definition"
     │   └── mocks
-    │       └── ERC2981Mock — "ERC2981 Module Reference Implementation"
+    │       ├── ERC2981Mock — "ERC2981 Module Reference Implementation"
+    │       └── ERC4626Mock — "ERC4626 Module Reference Implementation"
     ├── governance
     │   ├── TimelockController — "Multi-Role-Based Timelock Controller Reference Implementation"
     │   └── mocks