Refactor to single source file?

[BartMassey]

If you look at my merge-types branch you will find a bunch of macro work to replace the multiple implementation files with a single common one. Benchmarking indicates performance is about the same, as one would expect since I tried hard to generate the same code as before. The macro approach is not the most beautiful, but I despaired of playing the right games to get this performant with traits given the current state of stable Rust.

I didn't put in a PR because I didn't know if you wanted to go this route. I will if you like.