Remove const_fn(nightly) in favor of custom cfgs

[taiki-e]

As said in #17 (comment), to use the nightly only unstable feature, it needs to enable the feature at crate-level, and in any case, it needs build-script. So, this may not be very useful in practice.

This allows you to allow cfgs to be passed directly to const_fn.

- #[const_fn(cfg(my_cfg))]
+ #[const_fn(my_cfg)]

- #[const_fn(cfg(all(my_cfg1, my_cfg2)))]
+ #[const_fn(all(my_cfg1, my_cfg2))]