improve testing/code coverage

[folkertdev]

we should obviously take what we can from zlib/zlib-ng test suites, but they (especially for deflate) seem to rely on fuzzing and golden (end-to-end) tests. We should also have unit tests for internal logic (if only to make refactoring easier).

our codecov

Generally the coverage percentage looks allright, but many branches for edge cases aren't currently hit.

• inflate
• deflate
  • fast
  • quick
  • medium
  • slow
  • rte
  • huffman