Store ParseErr on heap

[JSAbrahams]

Relevant issues

• Partially resolves #396

Summary

This is because ParseErr is relatively large. Now it is stored on the heap, and the compiler also doesn't have to create extra space for the Ok value.

[codecov[bot]]

Codecov Report

Merging #406 (37dfff6) into develop (dbeb53e) will decrease coverage by 0.01%. The diff coverage is 31.11%.

@@             Coverage Diff             @@
##           develop     #406      +/-   ##
===========================================
- Coverage    87.56%   87.54%   -0.02%     
===========================================
  Files          109      109              
  Lines        12035    12016      -19     
===========================================
- Hits         10538    10519      -19     
  Misses        1497     1497              

Impacted Files	Coverage Δ
src/parse/class.rs	88.15% <0.00%> (ø)
src/parse/collection.rs	87.33% <0.00%> (ø)
src/parse/control_flow_stmt.rs	87.68% <0.00%> (ø)
src/parse/result.rs	43.13% <0.00%> (ø)
src/parse/statement.rs	91.33% <0.00%> (ø)
src/parse/definition.rs	93.68% <14.28%> (ø)
src/parse/iterator.rs	95.61% <50.00%> (ø)
src/parse/mod.rs	71.42% <50.00%> (-3.58%)	:arrow_down:
src/parse/ty.rs	96.73% <60.00%> (ø)
src/parse/control_flow_expr.rs	93.19% <66.66%> (ø)
... and 8 more