fast_block_t<N> is the fastest fixed-width block of at least N bits in one, by the thresholds of
least_block_t: uint_fast8_t where N <= 8, uint_fast16_t where N <= 16, uint_fast32_t where N <= 32,
and uint_fast64_t otherwise.
bit_fast<Owner>:
Constraints:remove_cv_t<Owner> is a resizable owner.
Result:bit_rebind<fast_block_t<N>, Owner>, where N is Owner's width.
bit_fast<basic_bit_array<uint64_t, 9>> is basic_bit_array<uint_fast16_t, 9>. bit_set is turned away.
Remarks: The platform decides how wide each uint_fastN_t is, so bit_fast names a block and promises no size.