Remove mutable ConstrainedValue

[Protryon]

Relevant checks are handled by ASG and wrapper/modifier Enum pattern causes bugs. Fixes #583. Fixes #449.

[codecov]

Codecov Report

Merging #601 (abee766) into master (2657c44) will increase coverage by 0.15%.
The diff coverage is 96.15%.

@@            Coverage Diff             @@
##           master     #601      +/-   ##
==========================================
+ Coverage   74.99%   75.14%   +0.15%     
==========================================
  Files         518      517       -1     
  Lines       16050    16021      -29     
==========================================
+ Hits        12036    12039       +3     
+ Misses       4014     3982      -32     

Impacted Files	Coverage Δ
compiler/src/value/value.rs	53.21% <ø> (-0.96%)	⬇️
compiler/src/statement/assign/assignee.rs	73.03% <83.33%> (-0.93%)	⬇️
compiler/src/definition/definition.rs	100.00% <100.00%> (ø)
compiler/src/expression/array/access.rs	85.71% <100.00%> (ø)
compiler/src/expression/array/index.rs	80.00% <100.00%> (ø)
compiler/src/expression/binary/binary.rs	100.00% <100.00%> (ø)
compiler/src/expression/circuit/access.rs	46.15% <100.00%> (ø)
compiler/src/expression/conditional/conditional.rs	92.85% <100.00%> (ø)
compiler/src/expression/expression.rs	94.02% <100.00%> (+1.49%)	⬆️
compiler/src/expression/tuple/access.rs	77.77% <100.00%> (ø)
... and 10 more

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