Remove unused ConstrVariant

[JSAbrahams]

Relevant issues

• Streamlines #439

Summary

Instead commit to the parent-child heuristic we have used anyway up until this point. Turns out that we don't need to switch around parent child in constraints, we just need better substitution rules, such as in #438 .

[codecov[bot]]

Codecov Report

Merging #440 (58c3f74) into develop (7d06ae2) will increase coverage by 0.02%. The diff coverage is 81.48%.

@@             Coverage Diff             @@
##           develop     #440      +/-   ##
===========================================
+ Coverage    88.59%   88.62%   +0.02%     
===========================================
  Files          110      110              
  Lines        11956    11936      -20     
===========================================
- Hits         10593    10578      -15     
+ Misses        1363     1358       -5     

Impacted Files	Coverage Δ
src/check/constrain/constraint/iterator.rs	84.61% <0.00%> (ø)
src/check/constrain/unify/function.rs	78.83% <66.66%> (+0.57%)	:arrow_up:
src/check/constrain/unify/ty.rs	76.13% <80.00%> (-0.46%)	:arrow_down:
src/check/constrain/constraint/mod.rs	91.89% <87.50%> (+4.13%)	:arrow_up:
src/check/constrain/constraint/builder.rs	96.50% <100.00%> (ø)
src/check/constrain/unify/expression/mod.rs	85.00% <100.00%> (-0.72%)	:arrow_down:
src/check/constrain/unify/expression/substitute.rs	100.00% <100.00%> (ø)
src/check/constrain/unify/link.rs	93.54% <100.00%> (ø)