Implement `FromStr` for `AST`

[JSAbrahams]

Relevant issues

• Resolves #455

Summary

This is more idiomatic Rust.

[codecov[bot]]

Codecov Report

Merging #457 (9f0fd3a) into develop (d8e4a9b) will increase coverage by 0.01%. The diff coverage is 94.95%.

@@             Coverage Diff             @@
##           develop     #457      +/-   ##
===========================================
+ Coverage    88.20%   88.22%   +0.01%     
===========================================
  Files          105      105              
  Lines        11501    11503       +2     
===========================================
+ Hits         10145    10148       +3     
+ Misses        1356     1355       -1     

Impacted Files	Coverage Δ
src/check/mod.rs	69.38% <66.66%> (ø)
src/parse/mod.rs	70.00% <73.33%> (ø)
src/check/constrain/generate/collection.rs	97.53% <100.00%> (ø)
src/check/constrain/mod.rs	100.00% <100.00%> (ø)
src/check/context/mod.rs	97.65% <100.00%> (+0.75%)	:arrow_up:
src/lib.rs	86.15% <100.00%> (-0.11%)	:arrow_down:
src/parse/call.rs	93.91% <100.00%> (ø)
src/parse/class.rs	88.09% <100.00%> (ø)
src/parse/collection.rs	94.91% <100.00%> (ø)
src/parse/control_flow_expr.rs	92.95% <100.00%> (ø)
... and 7 more