Fixtype of roles of integer bits in unsigned integers.
This is a tagged union type, introduced by fty::deftagsum.
Each bit in the object representation of unsigned integers [C17:6.2.6.2/1]
is either a value bit (representing a power of 2) or a padding bit.
This fixtype represents these choices,
where the natural number in the
This is similar to sinteger-bit-role, without the choice of a sign bit.