Add const_fn feature (more granular than nigthly)

[niklasf]

min_const_fn has been stabilized. It should still be opt-in, because it bumps the minimum Rust version to 1.31 (currently in beta).

This adds a new const_fn Cargo feature that allows using this without all current and future features implied by nightly. nightly still also implies const_fn.

[niklasf]

Build failed due to:

error: trait bounds other than `Sized` on const fn parameters are unstable
   --> lock_api/src/rwlock.rs:236:6
    |
236 | impl<R: RawRwLock, T> RwLock<R, T> {
    |      ^

Looks like min_const_fn is not enough.