MRG: fix exponential time explosion in sig check

[ctb]

Fixes #2646.

Embarrassing "let's make things run really slow" typo of the month... fixed!

In brief, the following code in manifest.py was causing an exponential explosion in time cost:

    def _add_rows(self, rows):
        self.rows.extend(rows)

        # maintain a fast check for md5sums for __contains__ check.
        md5set = self._md5_set
        for row in self.rows:
            md5set.add(row['md5'])

when called many times on a single row, because each addition of a row triggered an iteration over ALL rows.

This PR changes things so that we do for row in rows:.

Additional complexity => new tests to deal explicitly with the case where rows is a generator (which it turns out it often is, breaking dozens of tests when not accounted for)

[codecov]

Codecov Report

Merging #2762 (e3c8d58) into latest (6df0360) will increase coverage by 6.92%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           latest    #2762      +/-   ##
==========================================
+ Coverage   85.92%   92.85%   +6.92%     
==========================================
  Files         130      104      -26     
  Lines       14808    12423    -2385     
  Branches     2619     2621       +2     
==========================================
- Hits        12724    11535    -1189     
+ Misses       1784      587    -1197     
- Partials      300      301       +1     

Flag	Coverage Δ
hypothesis-py	25.81% <33.33%> (-0.01%)	⬇️
python	92.85% <66.66%> (-0.02%)	⬇️
rust	?

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

Files Changed	Coverage Δ
src/sourmash/sig/__main__.py	93.82% <0.00%> (-0.23%)	⬇️
src/sourmash/manifest.py	95.07% <100.00%> (+0.04%)	⬆️

... and 26 files with indirect coverage changes

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[ctb]

This should be ready for review @bluegenes

[bluegenes]

🎉

[ctb]

Low code coverage caused by debug code. We're all good here :).