Ensure all collections are hashable

[berquist]

Removing attrs dependence in #38 meant that hashes for ImmutableMultiDicts were no longer automatically computed. The implementation is the same as in #40.

[codecov]

Codecov Report

Merging #43 into master will increase coverage by 0.16%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #43      +/-   ##
==========================================
+ Coverage   90.07%   90.23%   +0.16%     
==========================================
  Files           7        7              
  Lines         665      676      +11     
==========================================
+ Hits          599      610      +11     
  Misses         66       66

Impacted Files	Coverage Δ
immutablecollections/_immutablemultidict.py	91.76% <100%> (+0.35%)	⬆️

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