[MRG] Refactor/expand MinHash add, iadd, and merge functionality.

[ctb]

This PR does a few things -

• first and foremost, it fixes some funky and useless merge tests in test__minhash.py. These tests were relying on merge being in-place, but were written as if they were not.
• second, it changes MinHash.merge to be only in-place, with no return value
• third, it adds an __add__ method.
• fourth, it provides some simple and very explicit tests of add, iadd, and merge.

Fixes #1211

TODO:

• create issue to consider renaming rust merge function
• create issue to deprecate in 3.5

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 #1282 (3b4a876) into latest (bc87da7) will increase coverage by 5.26%.
The diff coverage is 97.14%.

@@            Coverage Diff             @@
##           latest    #1282      +/-   ##
==========================================
+ Coverage   88.70%   93.96%   +5.26%     
==========================================
  Files         125       98      -27     
  Lines       18296    14758    -3538     
  Branches     1437     1438       +1     
==========================================
- Hits        16229    13868    -2361     
+ Misses       1822      643    -1179     
- Partials      245      247       +2     

Flag	Coverage Δ
python	93.96% <97.14%> (+<0.01%)	⬆️
rust	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
src/sourmash/minhash.py	93.30% <72.72%> (-1.24%)	⬇️
tests/test__minhash.py	99.31% <100.00%> (+0.05%)	⬆️
src/core/src/errors.rs
src/core/src/ffi/hyperloglog.rs
src/core/src/ffi/cmd/compute.rs
src/core/src/ffi/mod.rs
src/core/src/from.rs
src/core/src/encodings.rs
src/core/src/index/sbt/mod.rs
src/core/src/index/storage.rs
... and 19 more

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[ctb]

Ready for review @luizirber