Create lint reporting unnecessary casts

This is only a vague idea. I'm not sure it's strictly necessary, but...

Because we have so many integer types and because we switch between them quite often over time (eg. `Uint` -> `U256`), there could be many places in our codebase where a cast (of the form `b = Uint(0); a: U256 = U256(b)`, not with [`cast()`]) used to be necessary but is no longer required.

Having extra casts lingering around our codebase makes type checking less useful.

[`cast()`]: https://docs.python.org/3/library/typing.html#typing.cast