GAT std

A variant of Rust std traits that use GATs, as well as a macro to allow rewriting code to use these traits instead of the std equivalents.

Why?

1. These traits provide a common base so crates can all use the same definitions, like with num-traits
2. std likely won't be able to change to use these traits for quite a while, if ever. This allows users to take advantage of GATs in their code smoothly despite that.