Implement the $let operator.

[pcorpet]

Implements #536

[codecov]

Codecov Report

Merging #698 (4519556) into develop (1fe09be) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff            @@
##           develop     #698   +/-   ##
========================================
  Coverage    95.38%   95.38%           
========================================
  Files           19       19           
  Lines         3617     3639   +22     
========================================
+ Hits          3450     3471   +21     
- Misses         167      168    +1     

Impacted Files	Coverage Δ
mongomock/aggregate.py	97.19% <100.00%> (-0.07%)	⬇️
mongomock/database.py	98.54% <0.00%> (ø)
mongomock/filtering.py	99.00% <0.00%> (+0.03%)	⬆️

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