Generate identifiers from conditions of match arms

[JSAbrahams]

Summary

Match arms treated same way as if arms:

• For each new arm create new constraint set
• Use similar logic to for loop variable initialisation.

In future should have way to have some sort of union between constraints, much like env. This allows us to type check all possible execution paths.

When we implementing pattern matching this system will need to be extended.

Added Tests

Happy

• Variable only declared in condition of match arm usable in arm body.

Sad

• Shadow system does not delete outer variables of match. Started writing this while creating system of environment unions between match body arms. However, I don't think that we actually want this behaviour.

[codecov[bot]]

Codecov Report

Merging #380 (7db045b) into develop (63cc218) will increase coverage by 0.06%. The diff coverage is 100.00%.

@@             Coverage Diff             @@
##           develop     #380      +/-   ##
===========================================
+ Coverage    87.00%   87.06%   +0.06%     
===========================================
  Files          109      109              
  Lines        11950    12014      +64     
===========================================
+ Hits         10397    10460      +63     
- Misses        1553     1554       +1     

Impacted Files	Coverage Δ
src/check/constrain/generate/control_flow.rs	93.40% <100.00%> (+1.36%)	:arrow_up:
src/check/constrain/generate/expression.rs	94.28% <100.00%> (+1.42%)	:arrow_up:
src/check/constrain/generate/mod.rs	91.93% <100.00%> (ø)
src/parse/expr_or_stmt.rs	94.38% <0.00%> (-0.62%)	:arrow_down:
src/parse/ast/node.rs	79.77% <0.00%> (-0.11%)	:arrow_down:
src/check/ast/mod.rs	93.69% <0.00%> (-0.06%)	:arrow_down:
src/check/context/clss/generic.rs	93.15% <0.00%> (-0.04%)	:arrow_down:
src/generate/ast/mod.rs	92.82% <0.00%> (-0.04%)	:arrow_down:
src/check/ast/node.rs	95.75% <0.00%> (-0.02%)	:arrow_down:
src/parse/block.rs	100.00% <0.00%> (ø)
... and 9 more