A mixture of fuzz-testing improvements

Fix rans_set_cpu to work a second time. This only really matters for test/entropy.
Add a test/entropy_fuzz.c (@daviesrob you'll want this commit as it has the Makefile work, but you may want to edit the entropy_fuzz.c file itself. Mine is just a copy of entropy.c, which is working and finding bugs)
Fix the rans_set_cpu to once again support different CPU types for encoder and decoder. We do test every codec, but we accidentally lost this ability to have half and half, which affected cross-validation between codecs.
Small typo fix in entropy.c where it wasn't using RLE+PACK+SIMD.

samtools / htscodecs