Lint on uses of XOR as if it were POW: `2 ^ 32`

[estebank]

Given 2^32 is a common enough mistake to make, to the point that GCC is adding a lint for it, we probably should lint 2 ^ 8, 2 ^ 16, 2 ^ 32 and 2 ^ 64 as well and suggest 2u64.pow(x) or even better std::u64::MAX.

[jonas-schievink]

rust-lang/rust-clippy#4213 implements this in clippy

[nagisa]

This does not really pass the bar for no-false-positive-lints-in-rustc and is also RFC material.

[estebank]

Closing in lieu of the clippy lint