Remove `bit::stl::byte`

As discovered in the [`bit-memory`](https://github.com/bitwizeshift/bit-memory/commit/c63bd3312994c91b7592d629492e864025156045) library, `gsl::byte` (which `bit::stl::byte` is based on) violates c++ strict-aliasing when using it to alias in place of a `char*`. C++17 fixes this with `std::byte` and additional wording to single-unit fundamental types -- but without C++17, the `byte` type is useless.

Any uses of `bit::stl::byte` will require updating as well, such as `bit::stl::span`