Fixtype of decimal 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 decimal-floating-constant in the grammar in [C], but without 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 three possibilities of (i) a (decimal) fractional significand without an exponent, (ii) a (decimal) fractional significand with a (decimal) exponent, and (iii) a (decimal) integer significand with a (decimal) exponent. The first two possibilities are modeled as a fractional significand with an optional exponent.