Remove nightly requirement and add tests

[mqudsi]

The scary #![allow(incomplete_features)] and nightly-only #![feature(adt_const_params, const_fn_floating_point_arithmetic, generic_const_exprs)] attributes are no longer necessary. The remainder of the crate compiles as-is.

I've also adapted examples/main.rs into a proper test, added as src/tests.rs included only in the #[cfg(test)] case.