Streamline interfaces

[JSAbrahams]

Relevant issues

Working towards #279

Summary

• Add ASTTy class, which in future should represent an annotated type tree.
• Interfaces no longer take source and path as input (expect parse, which takes source as input of course). Instead, the user can opt to add these to the errors if they so choose.

Added Tests

[codecov[bot]]

Codecov Report

Merging #306 (a3d7612) into develop (555d94c) will decrease coverage by 0.25%. The diff coverage is 93.52%.

@@             Coverage Diff             @@
##           develop     #306      +/-   ##
===========================================
- Coverage    86.34%   86.09%   -0.26%     
===========================================
  Files          100      101       +1     
  Lines        10036    10038       +2     
===========================================
- Hits          8666     8642      -24     
- Misses        1370     1396      +26     

Impacted Files	Coverage Δ
src/generate/result.rs	23.33% <0.00%> (ø)
src/parse/mod.rs	88.88% <ø> (-2.03%)	:arrow_down:
src/parse/result.rs	66.12% <0.00%> (ø)
src/io.rs	92.10% <75.00%> (+2.10%)	:arrow_up:
src/check/mod.rs	85.00% <83.33%> (+8.07%)	:arrow_up:
src/lib.rs	86.36% <94.11%> (+6.03%)	:arrow_up:
src/check/ast.rs	100.00% <100.00%> (ø)
src/check/context/generic.rs	95.23% <100.00%> (+7.35%)	:arrow_up:
src/check/context/mod.rs	56.08% <100.00%> (ø)
src/check/result.rs	71.60% <100.00%> (-1.24%)	:arrow_down:
... and 15 more

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