 Add macros `gsl_DEFINE_ENUM_[BITMASK|RELATIONAL]_OPERATORS()`
 Add macros `gsl_DEFINE_ENUM_[BITMASK|RELATIONAL]_OPERATORS()`

* Add macros `gsl_DEFINE_ENUM_BITMASK_OPERATORS()` and `gsl_DEFINE_ENUM_RELATIONAL_OPERATORS()`

Use them to simplify the definition of `gsl::byte` in the sane case.
martinmoene/gsl-lite,martinmoene/gsl-lite,martinmoene/gsl-lite