Fix #19 - Remove broken Rust nightly support.

[finnbear]

This PR removes Rust nightly support, as a crude solution to the problem of changing feature flags reported in #19.

It can be applied immediately in non-crates.io-published dependents via:

# insert somewhere in your Cargo.toml
[patch.crates-io]
bitintr = { git = "https://github.com/finnbear/bitintr", branch = "remove_nightly" }

As far as I can tell, nightly wasn't used for very many things in the crate.

Feel free to ignore this and come up with a better solution!