Consolidate fields of decompressor

[anforowicz]

@fintelia, can you PTAL?

I hope that these 4 commits are mostly non-controversial (or at least much less controversial than the incomplete follow-up here for experimenting with dynamic table sizes).

This PR is motivated by the desire to experiment (in a separate follow-up, just in my local repo for now) with moving fn read_compressed into an instance method of CompressedBlock (currently this is a method of Decompressor). And making it a method of CompressedBlock would help with supporting various table sizes by making CompressedBlock a const-generic, while exposing fn read_compressed through a dyn CompressedBlockHandler trait.

I wondered whether to also change output_index parameter of fn read_compressed to &mut usize (and avoid returning the new/final value of this parameter). But I guess this would be unnecessary change = unnecessary risk (maybe this matters for performance). So let's just do the simplest thing for now?

Similarily, BitBuffer could be more encapsulated (e.g. living in a separate submodule), but again - this is not needed and at this point would seem like an unnecessary complication.

Note that in the 4th/final commit I can replace break with return Ok(...), because before break-ing we change self.state to something other than State::CompressedData - and the subsequent while and if check that self.state == State::CompressedData.