Partialbytes

Even though it seems like an enormous 5k lines PR, it isn't. 

The actual files we should be concerned with are:
- [ ] REAMDE.md
- [ ] include/libpolypasshash.h
- [ ] lib/libpolypasshash.c 
- [ ] tests/check_libpolypasshash.c
- [ ] tests/check_libpph_thresholdless.c
- [ ] tests/check_libpph_partialbytes.c