Improve dictionary decoder

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

Dictionary decoder has some hot paths in decoding

**Describe the solution you'd like**

We can unroll the dictionary gather/scatter loop to vectorize / improve throughput in this loop.
Also we can improve the codegen for `BitReader::get_value` by not going through a `slice`

**Additional context**

Start by locating the dictionary decoder and BitReader::get_value implementations, then inspect the dictionary gather/scatter loop and its existing benchmarks. Compare the current and revised code generation and throughput; done means the hot paths are measurably improved without changing decoding behavior.

Written by the indexing model from the issue text.