Implement Assignment Expressions (NamedExpr)

[Infinidoge]

Summary

Implements Python 3.8's Assignment Expressions (called NamedExpr within the ast

Checklist

PR Type

• [ ] This PR is a code change that implements a feature request.
• [ ] This PR fixes an issue.
• [x] This PR adds a new feature that is not an open feature request.
• [ ] This PR is not a code change (e.g. documentation, README, ...)

  Other

• [ ] This PR is a breaking change (e.g. methods or parameters removed/renamed)
• [x] If code changes were made then they have been tested.
• [ ] I have updated the documentation to reflect the changes.

[codecov[bot]]

Codecov Report

Merging #1 into master will increase coverage by 0.03%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master       #1      +/-   ##
==========================================
+ Coverage   99.05%   99.09%   +0.03%     
==========================================
  Files          10       10              
  Lines        1275     1322      +47     
==========================================
+ Hits         1263     1310      +47     
  Misses         12       12              

Impacted Files	Coverage Δ
draconic/interpreter.py	97.91% <100.00%> (+0.03%)	:arrow_up:
tests/test_executors.py	100.00% <100.00%> (ø)
tests/test_language.py	100.00% <100.00%> (ø)

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