Manually implement PartialEq for Option<T> and specialize non-nullable types This PR manually implements `PartialEq` and `StructuralPartialEq` for `Option`, which seems to produce slightly better codegen than the automatically derived implementation. It also allows specializing on the `core::num::NonZero*` and `core::ptr::NonNull` types, taking advantage of the niche optimization by transmuting the `Option<T>` to `T` to be compared directly, which can be done in just two instructions. A comparison of the original, new and specialized code generation is available [here](https://godbolt.org/z/dE4jxdYsa).