Optimize KmerMinHash add_sequence

[camillescott]

Removes a bunch of unnecessary string copying and adds some "unsafe" methods for hashing and sanity checking which accept C-strings (const char *) instead of std::string.

Note that this makes KmerMinHash::add_sequence volatile: the sequence is no longer passed as const. However, the user-facing interface always makes a copy of the incoming Python str in the Cython layer, so this volatility is discarded.

• 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 #744 into master will decrease coverage by <.01%.
The diff coverage is 95.65%.

@@            Coverage Diff             @@
##           master     #744      +/-   ##
==========================================
- Coverage    88.6%   88.59%   -0.01%     
==========================================
  Files          30       30              
  Lines        4722     4727       +5     
  Branches       45       46       +1     
==========================================
+ Hits         4184     4188       +4     
  Misses        536      536              
- Partials        2        3       +1

Impacted Files	Coverage Δ
sourmash/kmer_min_hash.hh	98.05% <95.65%> (-0.45%)	⬇️

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[camillescott]

ready to rumble @luizirber @ctb

[ctb]

LGTM!