Bring in some C++20 template language in flags.h

[sebjameswml]

From morph/flags.h:

The (more readable) C++20 equivalent to the C++17 compatible template declaration:

template <typename E, typename std::enable_if<std::is_enum<E>{}, bool>::type = true>

is:

template <typename E>
requires std::is_enum_v<E>

can it be one line?

template <typename E> requires std::is_enum_v<E>

Anyhow - do the replacement in flags.h after merging the constexpr code.