Compression of repdef buffers in mini-block encoding

[westonpace]

We currently don't compress rep-def buffers at all when mini-block encoding. This is pretty wasteful as it requires 4-bytes per value. We should, at the very least, make sure we are byte packing and, for mini-block, it would probably be suitable to use bitpacking.

I think the main challenge for bit-packing is that we only have the "bitpack in 1024-element chunks" variant of bit-packing and not a simple "bitpack this buffer" interface.

This is not strictly required for the beta and could be done as part of the stabilization.