Optimize integer division by a constant in several cases.

Optimize integer division by a constant in several cases.

Convert integer division by a constant into multiplication to gain
a large speedup as the machine instructions for multiplication are
typically executed much faster than those for division.

This is implemented using a deftransform on TRUNCATE that triggers
if the dividend is known to fit in an unsigned machine word and if
the divisor is a constant, also fitting in an unsigned machine word.
(The cases that are optimized by other existing transforms, for example
if the divisor is a power of two, are left to these transforms.)

The replacement code is based on a widening multiply (that is already
available as bignum calculations need it) and possibly some shifts and
an addition to calculate the quotient. If the remainder is needed,
additionally a (normal) multiplication and a subtraction are generated.

As several other integer division operations are implemented using
TRUNCATE, this also affects CEILING, FLOOR, MOD and REM with the same
argument types. CEILING and FLOOR, however, are optimized only when
SAFETY=0 since they are declared MAYBE-INLINE.