Adding missing __len__ method to dict objects

[kellrott]

Old behavior:

len({"a" : 1, "b" : 2})

TypeError: "object of type 'dict' has no len()"

New behavior returns the number of elements in the dict

[codecov]

Codecov Report

Merging #171 (849af0e) into master (f712a5d) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #171   +/-   ##
=======================================
  Coverage   72.89%   72.90%           
=======================================
  Files          71       71           
  Lines       11682    11683    +1     
=======================================
+ Hits         8516     8517    +1     
  Misses       2595     2595           
  Partials      571      571           

Impacted Files	Coverage Δ
py/dict.go	77.55% <100.00%> (+0.23%)	⬆️

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[sbinet]

LGTM