Add feature when compiling in nightly & include generic_const_exprs if that feature is enabled

[coreylowman]

I'd imagine you could do something like use cfg tags to check if they're on nightly. Something like this: https://stackoverflow.com/questions/49247618/check-whether-compiler-is-nightly-at-compile-time

that way no other features need to be enabled, it just works on nightly.

Originally posted by @jafioti in #43 (comment)