Fix the DEFTRANSFORM of RANDOM for hairy integer types.

Fix the DEFTRANSFORM of RANDOM for hairy integer types.

With integer types that are neither an interval nor a single known value
the DEFTRANSFORM used to generate an expression that had two problems:
First, it yielded very uneven distributions of random values for most
arguments to RANDOM that are not very small. Second, it used a too small
RANDOM-CHUNK under 64 bits word size thus never generating numbers
larger than (1- (EXPT 2 32)) even if RANDOM's argument was larger than
(EXPT 2 32).

Fix this by giving up the transform in these cases.

Add a new file "tests/random.pure.lisp" containing tests for this.