Add comments and examples to the ceilinged/floored mul-by-frac generator.
Remove the SIGNED-P argument from GEN-MULTIPLY-SHIFT-EXPR.

Use the signs of the arguments instead to determine whether
unsigned or signed multiplication should be used.
This makes some of the generated code simpler, since unsigned
words have a bigger range than positive signed words.
Add transform for a trivial case of integer-rational division.

This converts division by 1/Y to multiplication. It is handled
separately because the remainder is always 0, so it doesn't have
to be calculated.