Clean up our wild use of conditional compilation.

[tobz]

Conditional compilation is a necessity for targeting different platforms, although at this point, we've found ourselves down quite the rabbit hole in terms of how Cargo.toml and src/monotonic.rs look.

We should spend some time trying to clean this up where we can. On the code side, we might be better off splitting out the various architecture-specific implementations of Monotonic into dedicated files. As far as Cargo.toml, that one is dicey, but perhaps there's some magic we can exploit there to do it in a cleaner fashion.