Fixtype of roles of integer bits in signed integers.
This is a tagged union type, introduced by fty::deftagsum.
Each bit in the object representation of signed integers [C17:6.2.6.2/2]
is either a value bit (representing a power of 2)
or a padding bit
or a sign bit.
This fixtype represents these choices,
where the natural number in the
This is similar to uinteger-bit-role, with the added choice of a sign bit.