Faster fingerprints

[kmike]

This should fix #65.

For a nested dict with 30Kb string value _fast_hash is about 3-4x faster than _json_based_hash(_process(value, sha=True)), and the latter is about 3-4x faster than _json_based_hash(value). Because with cache_args it is expected to have a very large amount of duplicates I went with this complicated solution.

[codecov-io]

Current coverage is 89.96%

Merging #68 into master will increase coverage by +0.36% as of 8bf3a9a

@@            master     #68   diff @@
======================================
  Files            9       9       
  Stmts          529     548    +19
  Branches       100     107     +7
  Methods          0       0       
======================================
+ Hit            474     493    +19
  Partial         26      26       
  Missed          29      29       

Review entire Coverage Diff as of 8bf3a9a

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