[MRG] define get_mins in terms of .hashes, not the other way around

[luizirber]

This PR implements .get_mins and .get_hashes in terms of .hashes, as well as replacing their usage in tests by:

• .get_mins() ->.hashes.keys()
• .get_mins(with_abundance=True) -> .hashes
• .get_hashes() -> .hashes.keys()

(it also brings down the warnings count from 3975 to 1908...)

Checklist

• Is it mergeable?
• make test Did it pass the tests?
• make coverage Is the new code covered?
• Did it change the command-line interface? Only additions are allowed
  without a major version increment. Changing file formats also requires a
  major version number increment.
• Was a spellchecker run on the source code and documentation after
  changes were made?

[codecov]

Codecov Report

Merging #1154 into latest will increase coverage by 9.23%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           latest    #1154      +/-   ##
==========================================
+ Coverage   83.72%   92.95%   +9.23%     
==========================================
  Files          95       71      -24     
  Lines        9006     5693    -3313     
==========================================
- Hits         7540     5292    -2248     
+ Misses       1466      401    -1065     

Flag	Coverage Δ
#rusttests	?

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Impacted Files	Coverage Δ
sourmash/minhash.py	97.27% <100.00%> (+0.01%)	⬆️
sourmash/search.py	93.57% <100.00%> (ø)
src/core/src/cmd.rs
src/core/src/sketch/ukhs.rs
src/core/src/index/linear.rs
src/core/src/sketch/nodegraph.rs
src/core/src/index/mod.rs
src/core/src/ffi/utils.rs
src/core/src/signature.rs
src/core/src/wasm.rs
... and 16 more

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[luizirber]

Ready for review and merge @ctb

Note that asv benchmarks are failing because .get_mins is slower now. I'm OK with that, because we should be avoiding using .hashes in internal sourmash code anyway...

[ctb]

check removal of deprecated bits of code in 4.0? otherwise looks good!