Use a 32-bit limb size ("like a digit, but bigger") for floating-point
conversion. This cuts the number of multiplications per constant by a
factor of four.
This means supporting fractional-limb-sized numbers, so while we're at
it, add support for 8-bit floating point numbers (apparently used in
graphics and in audio compression applications.)
Unify all the standard IEEE formats into one function, add support for
IEEE standard 128-bit floating point numbers.
The 80-bit format is still special since it explicitly represents the
integer portion.
