Fix non avx2 (#61)

* Fix non-AVX2 whitespace splitting.

This ended up being the root cause of #60. Most of this change is
refactoring to ensure that all available implementations run during
testing, not just AVX2. That'll ensure we can avoid regressions in the
future.

* Add coverage, start debugging generic CSV

* fix generic impl for csv quote masks

* refactor, test bytes splitter

* improve practices around runtime detection of cpu features

* different take on dynamic cpu feature selection

* add native back in to rust flags