More refactoring of MinHash comparison code

[ctb]

Fixes #868.

• create rust functions jaccard and angular_similarity to do the actual math on compatible signatures
• make Python compare and similarity call the same compare function on rust side
• refactor out duplicated code for signature downsampling to new rust function downsample_max_hash

I made the decision to just simplify and rationalize the rust side of things, because our Python API for MinHash objects (in sourmash/_minhash.py) probably shouldn't change without a major version release...

• 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 #882 into master will increase coverage by 27.83%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           master     #882       +/-   ##
===========================================
+ Coverage   50.23%   78.06%   +27.83%     
===========================================
  Files          25       95       +70     
  Lines        2365     7308     +4943     
===========================================
+ Hits         1188     5705     +4517     
- Misses       1177     1603      +426

Flag	Coverage Δ
#rusttests	50.23% <ø> (ø)	⬆️

Impacted Files	Coverage Δ
sourmash/commands.py	84.4% <100%> (ø)
sourmash/cli/compare.py	100% <100%> (ø)
sourmash/sbt.py	86.52% <100%> (ø)
sourmash/logging.py	96.07% <0%> (ø)
sourmash/__main__.py	92.3% <0%> (ø)
sourmash/_minhash.py	96.2% <0%> (ø)
sourmash/cli/lca/classify.py	100% <0%> (ø)
sourmash/cli/storage/__init__.py	100% <0%> (ø)
sourmash/lca/command_summarize.py	79.26% <0%> (ø)
sourmash/sbt_storage.py	86.36% <0%> (ø)
... and 63 more

---

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[ctb]

Ready for review @luizirber, and thx for the suggestion, @kloetzl !

[kloetzl]

Uh, nice. That really reduced the line count. Less lines, less bugs!

[ctb]

On Sun, Feb 02, 2020 at 05:36:17AM -0800, Fabian Klötzl wrote: Uh, nice. That really reduced the line count. Less lines, less bugs!

yep, thank you!

[ctb]

OK I think this is ready.

[luizirber]

I'll fix the wasm-pack issue. Should I also fix all the failing Rust tests?