Add in construct for in operation

[JSAbrahams]

Relevant issues

Fixes #113

Summary

Allows us to use Python's in boolean operator.

Added Tests

Check that in operator works as expected.

[codecov[bot]]

Codecov Report

Merging #116 into master will increase coverage by 0.03%. The diff coverage is 85.14%.

@@            Coverage Diff             @@
##           master     #116      +/-   ##
==========================================
+ Coverage    82.5%   82.54%   +0.03%     
==========================================
  Files          65       65              
  Lines        4539     4565      +26     
==========================================
+ Hits         3745     3768      +23     
- Misses        794      797       +3

Impacted Files	Coverage Δ
tests/desugar/error.rs	88.33% <ø> (+0.33%)	:arrow_up:
tests/parser/invalid/control_flow.rs	100% <ø> (ø)	:arrow_up:
src/core/construct.rs	27.64% <0%> (-0.23%)	:arrow_down:
src/parser/expression.rs	93.84% <100%> (+0.01%)	:arrow_up:
src/core/mod.rs	64.28% <100%> (-0.19%)	:arrow_down:
src/desugar/control_flow.rs	78.12% <100%> (-0.67%)	:arrow_down:
tests/parser/valid/operation.rs	100% <100%> (ø)	:arrow_up:
tests/desugar/control_flow.rs	89.65% <100%> (-0.43%)	:arrow_down:
src/parser/control_flow_stmt.rs	70.27% <100%> (-1.53%)	:arrow_down:
src/parser/ast.rs	25.94% <50%> (+3.65%)	:arrow_up:
... and 5 more

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