Add step to ranges

[JSAbrahams]

Relevant issues

89

Summary

Update range syntax so we can have steps in ranges, akin to Kotlin. Also change range structure to whether it is inclusive is a boolean stored within ASTNode::Range.

Added Tests

Tests for the new types or ranges:

• When parsing them
• When desugaring them

[codecov[bot]]

Codecov Report

Merging #92 into master will increase coverage by 0.17%. The diff coverage is 84.13%.

@@            Coverage Diff             @@
##           master      #92      +/-   ##
==========================================
+ Coverage   83.44%   83.62%   +0.17%     
==========================================
  Files          63       63              
  Lines        4113     4237     +124     
==========================================
+ Hits         3432     3543     +111     
- Misses        681      694      +13

Impacted Files	Coverage Δ
src/parser/ast.rs	24.82% <0%> (-0.18%)	:arrow_down:
src/core/construct.rs	33.66% <0%> (+1.01%)	:arrow_up:
src/parser/expression.rs	93.82% <100%> (+0.86%)	:arrow_up:
src/lexer/mod.rs	96.95% <100%> (+0.01%)	:arrow_up:
src/lexer/token.rs	89% <100%> (+1%)	:arrow_up:
src/core/mod.rs	70.44% <100%> (+0.76%)	:arrow_up:
src/desugar/node.rs	68.05% <83.33%> (-0.02%)	:arrow_down:
tests/parser/valid/control_flow.rs	86.69% <85%> (-0.39%)	:arrow_down:
tests/desugar/control_flow.rs	90.08% <87.8%> (-0.05%)	:arrow_down:
tests/parser/valid/expression_and_statement.rs	85.46% <92.3%> (+0.79%)	:arrow_up:
... and 2 more

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