Support for 'std::optional'

[jonathon-bell]

The function rc::gen::maybe lifts a generator into the Maybe monad, which can be quite useful.

C++17 finally introduced its own version of Maybe to the standard library, namely std::optional.

Would there be any interest here in a submission that provides support for this addition to the language for compilers that can support it? rc::gen::optional, perhaps? If so, I'd be happy to contribute.

Cheers,

Jonathon