Start implementing unsigned ceiling transformation.

TODO: Add comments, convince the compiler to use unsigned word
arithmetic.
Replace the examples in GEN-(UN)SIGNED-DIV-BY-CONSTANT-EXPR with tests.

This way, we'll notice if we forgot to update the examples after a change
in the generator code.
Add comments about the ceiling transform and make it use non-generic arithmetic.

Do the ADD-CEIL-EXTENSION part outside of GEN-UNSIGNED-DIV-BY-CONST-EXPR
so that the intermediate value can be used for calculating the remainder.
Remove unnecessary function and comment.

LD(X) isn't used in GEN-SIGNED-DIV-BY-CONST-EXPR, remove it.
Also remove comment in the truncate transform about handling
the case when the quotient is constant, as that was removed
in commit 140a70b.