Vectorised Pack Bits within BitWriter

**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**