bitpacking 0.8.0

Made Bitpacker1x, Bitpacker4x, and BitPacker8x optional via a feature flag ;
by default all 3 are enabled.

Compiling bitpacking was proven to consume a lot of RAM so it seems like a good idea to make it possible
for project using the bitpacking crate like tantivy to avoid compiling the unused BitPackers.

Tantivy 0.7

Bugfix #11
AVX2 bitpacking was broken. This bug does not affect tantivy.