[rational] make rational constructors noexcept

From: Frédéric (ufospoke_at_[hidden])
Date: 2019-08-06 05:46:55

Hi,

I think that some rational constructors can be marked noexcept.
In particular:
- default constructor: rational()
- constructor from integer: rational(const T& n, typename
enable_if_c<...>::type const* = 0)

Concerning the constructor with numerator and denominator, if the
denominator is always not null, it could be noexcept but I guess we
cannot enforce not null unless we use some kind of gsl::not_null.

Your thoughts?

Thanks,

F