bit_enum_set<Enum> is the fixed set of an enumeration's keys, one bit each in the smallest block that holds them:
basic_bit_fixed_set<Enum, least_block_t<S>, S, bit_key_mapping<Enum>> for S = bit_key_mapping<Enum>::size.
Constraints:Enum is an enumeration whose default mapping names a size: its values listed in enum_traits,
ended by a count sentinel, or a bit_key_mapping specialized directly.
basic_bit_fixed_set{E::a, E::b} deduces the same type. Another block is
basic_bit_fixed_set<Enum, Block, S>'s to spell.
An enumerator is the one-element set holding it, on either side of |, &, ^ and -, as for basic_bit_set.