Summary

Tracking Issue: #7216

Adds a new CompressionEstimate type in vortex-compressor/src/estimate.rs that the expected_compression_ratio method now returns. Additionally moves some things around for clarity.

Note that this is not just a refactor, there is subtle logic that has changed in a few places (that I think is better, not actually sure). I'm happy to split out some stuff into other PRs if that helps.

Future Work

• I also would like to add a variant called Exact that returns the fully compressed array in the case that we can only determine if a scheme is a candidate by compressing the whole thing without any errors; the only case where we want to do this is SequenceArray (and maybe there's an argument to do this for ConstantArray too, but the semantics around ConstantArray should be even more special regardless, imo).
• This might be in a ResolvedEstimate enum instead.
• There are also a bunch of TODOs littered everywhere that are easily fixed, but I want to do those in a followup.
• We probably want to hardcode the ConstantScheme logic into the compressor since I cannot think of any reason why you would not want to have a ConstantScheme (except when you have a very small array, and at that point you don't care about perf regardless).

API Changes

expected_compression_ratio now only takes stats and compressor context (it does not take the compressor at all) and returns a CompressionEstimate. This method must be super quick, and any sampling or expensive operations are now deferred to later by the compressor.

Testing

Just a few extra tests, am relying on the existing test suite as it's not like completely new logic is happening.