Hexadecimal float literals, for perl #122219

Hexadecimal float literals, for perl #122219

Do not use system strtod/strtold since its C99 (which we do not require).

If we can fit the mantissa bits of a NV to a UV (which is the case for
the most common format, 64-bit IEEE 754 double, which has 52 mantissa bits)
we will use that, and only in the end convert to NV.  If we cannot fit
the bits, accumulate the bits incrementally to a NV.