Use a 32-bit floating-point limb size; support 8-bit float

Use a 32-bit floating-point limb size; support 8-bit float

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.)