[WIP] add support for sourmash sketch fromfile functionality.

[ctb]

This PR updates command_sketch.py and command_compute.py to support new sourmash sketch fromfile functionality, currently being developed in the ctb/2022-sourmash-sketchfrom repo and also in #1885.

This is an ongoing PR that separates the support code changes from the fromfile command itself.

This PR:

• updates the ComputeParameters class to support __repr__ and __eq__.
• adds ComputeParameters.to_param_str method.
• refactors _signatures_for_sketch_factory to properly handle no default_moltype and produce one ComputeParameters object per ksize, along with general cleanup and refactoring.

Ref #1671

[codecov]

Codecov Report

Merging #1884 (07637c5) into latest (dbda4ef) will increase coverage by 7.93%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           latest    #1884      +/-   ##
==========================================
+ Coverage   82.62%   90.55%   +7.93%     
==========================================
  Files         121       91      -30     
  Lines       13109     8946    -4163     
  Branches     1756     1772      +16     
==========================================
- Hits        10831     8101    -2730     
+ Misses       2015      580    -1435     
- Partials      263      265       +2     

Flag	Coverage Δ
python	90.55% <91.66%> (-0.01%)	⬇️
rust	?

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Impacted Files	Coverage Δ
src/sourmash/index/__init__.py	96.45% <ø> (ø)
src/sourmash/command_compute.py	92.69% <86.66%> (-0.67%)	⬇️
src/sourmash/command_sketch.py	88.58% <100.00%> (+0.58%)	⬆️
src/core/src/encodings.rs
src/core/src/lib.rs
src/core/src/ffi/utils.rs
src/core/src/ffi/mod.rs
src/core/src/index/linear.rs
src/core/src/sketch/hyperloglog/estimators.rs
src/core/src/ffi/index/revindex.rs
... and 23 more

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[ctb]

@bluegenes @mr-eyes if you have space to review this, it's merge-able at any time. I am (trying to) cleanly separate out the internal changes needed for robust support of #1885, but since each bolus of changes is independently good, might be nice to merge in the work every so often.

[bluegenes]

i think this lgtm

[ctb]

thx - will wait for tests to finish and then merge! nice to keep the PRs from getting too big 😆