Fixtype of hexadecimal core floating constants [C:6.4.4.2] [C:A.1.5].
This is a tagged union type, introduced by fty::deftagsum.
This corresponds to hexadecimal-floating-constant in the grammar in [C], but without the prefix and the optional suffix, which is what we mean by `core' in the name of this fixtype (we factor things slightly differently in our fixtypes; see fconst). This covers the two possibilities of (i) a (hexadecimal) fractional significand with a (binary) exponent and (iii) a (hexadecimal integer significand with a (binary) exponent.