Add signed version of %MULTIPLY-AND-ADD and use it for division by rationals.

The new function is SB!KERNEL:%SIGNED-MULTIPLY-AND-ADD-HIGH. It multiplies
two args, adds a third arg to the product, and returns the high bits of the
sum.
Use it in GEN-SIGNED-MUL-BY-FRAC-EXPR for 2N-bit multiplication.

TODO: Add VOPs for architectures other than x86(-64).
Add some examples and minor fixes.

Add examples for the signed mul-by-frac generator and add some small fixes.
Change the examples in previous generators to make them more readable.
Change some constants in the div-by-mul tester for rational division.