Release 1.2.0

Several minor features have been added to this release, and a few
minor fixes.

This contains the following changes:

• Updates underlying_type to SFINAE if type is not an enum (#18)
• Adds is_[nothrow_]invocable_r, which was missing from previous
  release
• Fixes some inline visibility bugs surrounding optional
• Adds conditionally explicit span constructors (#21)
  • This is potentially a breaking change -- however this is not bumping
    the major version of this library since the previous implementation
    was following the experimental API of span -- which was not the
    final version
• Fixes /W4 warnings for MSVC