[MRG] make SourmashSignature printable and hashable

[ctb]

Add __str__, __repr__, and __hash__ to SourmashSignature objects.

• 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?

[ctb]

@luizirber @betatim ready for review & merge!

[codecov-io]

Codecov Report

Merging #332 into master will increase coverage by 0.05%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage    86.9%   86.96%   +0.05%     
==========================================
  Files          13       13              
  Lines        2009     2018       +9     
  Branches       36       36              
==========================================
+ Hits         1746     1755       +9     
  Misses        262      262              
  Partials        1        1

Impacted Files	Coverage Δ
sourmash_lib/signature.py	93.58% <100%> (+0.39%)	⬆️

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