Remove File Node

[JSAbrahams]

Summary

• Move logic to parse statements, everything is now a block. This means that imports may also be nested, though the context does not yet deal with this.
• Add EOF token
• Treat import as statement.
• Make Position and CaretPos implement copy. Then we don't have to constantly clone these simple constructs.
• Improve some error messages.

[codecov[bot]]

Codecov Report

Merging #325 (5c678a5) into develop (ed9a647) will decrease coverage by 0.27%. The diff coverage is 84.45%.

@@             Coverage Diff             @@
##           develop     #325      +/-   ##
===========================================
- Coverage    87.23%   86.95%   -0.28%     
===========================================
  Files          105      104       -1     
  Lines        11335    11424      +89     
===========================================
+ Hits          9888     9934      +46     
- Misses        1447     1490      +43     

Impacted Files	Coverage Δ
src/check/ast/node.rs	95.43% <ø> (-0.06%)	:arrow_down:
src/check/constrain/generate/mod.rs	90.16% <ø> (-0.16%)	:arrow_down:
src/check/constrain/generate/ty.rs	0.00% <0.00%> (ø)
src/check/name/truename/generic.rs	62.96% <0.00%> (ø)
src/check/constrain/unify/expression/mod.rs	80.55% <33.33%> (ø)
src/check/context/arg/generic.rs	72.83% <37.50%> (ø)
src/check/constrain/generate/statement.rs	81.25% <50.00%> (ø)
src/check/context/parent/generic.rs	78.57% <50.00%> (ø)
src/check/name/stringname/generic.rs	93.33% <50.00%> (ø)
src/generate/result.rs	23.33% <50.00%> (ø)
... and 72 more

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[JSAbrahams]

Decrease in coverage is largely due to simplification of logic.