Implement interfaces in mock tokens

[arr00]

Currently MockERC20 and MockERC721 don't implement their respective interfaces. This PR fixes that.

[mds1]

Thanks for your PR :)

What is the benefit of having them inherit from the interfaces? This feels like a nontrivial diff that adds code for unclear benefit, since these ERC interfaces are finalized and static, and the mocks are already compliant. I'd like to keep everything in forge-std as simple as possible so it's easy to read/maintain and to reduce compilation times.

I'm also not a fan of making the variables private since that limits devs from inheriting the mock and adding e.g. new methods to set approvals

[arr00]

It is unintuitive for there to be an interface as well as an implementation of that interface in the same repo but them not be tied together at all. I came across this when using the interface and the mock token in a situation shown below:

IERC20 fromToken;
fromToken = IERC20(address(new MockERC20()));

It is highly preferred for the below to work:

IERC20 fromToken;
fromToken = new MockERC20();

[mds1]

It is highly preferred for the below to work:

IERC20 fromToken;
fromToken = new MockERC20();

That would be really nice but I don't think that is supported since solidity doesn't have structural typing: https://forum.soliditylang.org/t/making-interfaces-structural/1790

import {MockERC20} from "../src/mocks/MockERC20.sol";
import {IERC20} from "../src/interfaces/IERC20.sol";
import {Test} from "../src/Test.sol";

contract ATest is Test {
    IERC20 public token;

    function setUp() public {
      IERC20 fromToken;
      fromToken = new MockERC20();
    }
}

$ forge build
[⠢] Compiling...
[⠊] Compiling 26 files with 0.8.23
[⠒] Solc 0.8.23 finished in 648.53ms
Error: 
Compiler run failed:
Warning (3420): Source file does not specify required compiler version! Consider adding "pragma solidity ^0.8.23;"
--> test/A.t.sol

Error (7407): Type contract MockERC20 is not implicitly convertible to expected type contract IERC20.
  --> test/A.t.sol:19:19:
   |
19 |       fromToken = new MockERC20();
   |                   ^^^^^^^^^^^^^^^

[arr00]

@mds1 Consider running that test in the context of this PR. It works.

[mds1]

Oops you're right, I was on the wrong branch. Ok, agreed that is a much better UX!

Two small changes then we can merge

[arr00]

@mds1 let me know if any other changes are needed

[mds1]

Thank you!