Add tests and comments for the integer-rational division.

Extend the div-by-mul test cases to test for unsigned division
of integers by ratios. Add comments and example expressions for
GEN-UNSIGNED-MUL-BY-FRAC-EXPR.
Fix incorrect range calculation in GEN-SIGNED-DIV-BY-CONSTANT-EXPR.
Implement 2n-bit multiply-and-shift in the mul-by-frac generator.

Taken from one of Paul Khuong's writing on div-by-mul.
TODO: Add comments explaining some steps of the algorithm.