Fixes assignment of mutable variable to mutable variable

[collinc97]

Motivation

Fixes #449

Test Plan

Test added for #449 example

[codecov]

Codecov Report

Merging #466 (4cb07d5) into master (ced0246) will increase coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #466      +/-   ##
==========================================
+ Coverage   73.46%   73.48%   +0.02%     
==========================================
  Files         453      453              
  Lines       13483    13489       +6     
==========================================
+ Hits         9905     9913       +8     
+ Misses       3578     3576       -2     

Impacted Files	Coverage Δ
compiler/src/value/value.rs	61.13% <100.00%> (+0.13%)	⬆️
compiler/tests/mutability/mod.rs	100.00% <100.00%> (ø)
type-inference/src/objects/frame.rs	89.70% <0.00%> (+0.42%)	⬆️

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