[MRG] Output added hashes up to the Python layer

[luizirber]

Spin-off #477 changes to the C++/Cython layer

• 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-io]

Codecov Report

Merging #482 into master will decrease coverage by 0.09%.
The diff coverage is 75%.

@@            Coverage Diff            @@
##           master     #482     +/-   ##
=========================================
- Coverage   90.75%   90.65%   -0.1%     
=========================================
  Files          33       33             
  Lines        5007     5019     +12     
  Branches       36       40      +4     
=========================================
+ Hits         4544     4550      +6     
- Misses        463      466      +3     
- Partials        0        3      +3

Impacted Files	Coverage Δ
sourmash/kmer_min_hash.hh	96.62% <75%> (-3.38%)	⬇️

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[luizirber]

Not used, and will go away with Rust merge anyway.