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.)
Add special operators to allow the use of floating-point constants in
contexts other than DW/DD/DQ/DT/DO.
As part of this checkin, make MAX_KEYWORD generated by tokhash.pl,
since it knows what all the keywords are so it can tell which one is
the longest.
