Vectorised Pack Bits within BitWriter

[tustvold]

Is your feature request related to a problem or challenge? Please describe what you are trying to do.

BitReader has a vectorised get_batch method for unpacking blocks of bit-packed values. This provides a significant performance improvement over processing each value separately.

BitWriter, however, has no corresponding functionality to write blocks of values, and this represents a major bottleneck for encoding primitive values.

Describe the solution you'd like

We should add a BitWriter::put_batch method that can vectorise packing runs of primitives

Describe alternatives you've considered

We could not do this

Additional context