WIP: implement RevIndexOps for mem_revindex, and implement generic CounterGather struct in Rust

[ctb]

NOTE: This is a PR into #3545

This PR mimics the Python layer by having RocksDB databases return a CounterGather object directly; doing this responsibly meant fully implementing RevIndexOps for RevIndex in memory, as well as fleshing out the whole Python FFI layer 🎉

Current benchmarks:

prefix	s	max_rss
fastmultigather_rocksdb	21.5	0.5
pygather_rocksdb+prefetch	101.1	0.9
fastgather	199.8	12.0
fastmultigather	910.9	12.8
pygather	1752.3	20.0
pygather_rocksdb-no-prefetch	3528.9	1.1

[codecov]

Codecov Report

Attention: Patch coverage is 72.22222% with 220 lines in your changes missing coverage. Please review.

Project coverage is 87.59%. Comparing base (a1565b0) to head (d8c34f4).
Report is 47 commits behind head on impl_revindex.

Files with missing lines	Patch %	Lines
src/core/src/ffi/index/revindex.rs	0.00%	114 Missing ⚠️
src/core/src/index/revindex/mem_revindex.rs	89.52%	35 Missing ⚠️
src/core/src/index/revindex/mod.rs	61.84%	29 Missing ⚠️
src/core/src/index/revindex/disk_revindex.rs	60.71%	22 Missing ⚠️
src/sourmash/index/revindex.py	88.88%	14 Missing and 6 partials ⚠️

Additional details and impacted files

@@                Coverage Diff                @@
##           impl_revindex    #3584      +/-   ##
=================================================
- Coverage          87.68%   87.59%   -0.10%     
=================================================
  Files                137      136       -1     
  Lines              22717    23043     +326     
  Branches            2280     2273       -7     
=================================================
+ Hits               19919    20184     +265     
- Misses              2473     2530      +57     
- Partials             325      329       +4     

Flag	Coverage Δ
hypothesis-py	25.38% <30.48%> (+0.07%)	⬆️
python	92.36% <89.30%> (+0.01%)	⬆️
rust	81.10% <66.94%> (-0.02%)	⬇️

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