Remove simd bitpacking implementations for high bit width

[fulmicoton]

For sorted stuff, having a super fast implementation is not necessary above bitwidth = 8. We can fallback to the scalar implementation.
For unsorted stuff, I think we can also restrict ourselves to bitwidth <= 8, although this is a trade-off.

[PSeitz]

The amount of generated code seems excessive currently

➜  bitpacking git:(master) cargo expand | wc -l

330128

[fulmicoton]

Definitely :)