Remove hooks from ERC20

[JulissaDantes]

Fixes #3535

The current way to customize token transfers(mint, burn, transfer) is by overriding the hooks _beforeTokenTransfer and _afterTokenTransfer. This refactor removes both hooks from the ERC20 contract, and implements the logic inside the transfer function, so future customizations only rely on overriding a single function.

PR Checklist

• Tests
• Documentation
• Changelog entry

[frangio]

Why is this removed?

[JulissaDantes]

Since everything goes through the transfer, it cannot know if its coming from mint or burn, so transfer has a check to ensure that from and to cannot be the zero address, but if a user sends a mint to address zero it would received both from and to as the zero address. There is a test for that case on ln 206

[Amxx]

Line 206 is testing that you cannot mint to address 0. That is a valid test, but its separate from this. This tests that you cannot call transfer to the address 0.

Tests are checking for behavior correctness. Its ok (and I think its needed) to have multiple tests that are resolved by the same require in different context.

I think this test should have been kept.

[Amxx]

I wonder if that is the cause of the coverage decrease.

[frangio]

There is a separate point we need to decide, which is whether the external transfer and transferFrom functions should accept to = address(0).

I think this PR should keep the current behavior, and so we need to add in those external functions the necessary require statements.