Store literal expression type in the AST

[jmackie]

This is in anticipation of type aliases (#62).

The motivating example is:

type alias WholeNumber = Int;

five : WholeNumber = 5;

Currently we would drop the alias type annotation as part of type checking, but we want to keep it around for more helpful type errors.

[codecov]

Codecov Report

Merging #90 (ed14bc9) into main (0477330) will increase coverage by 0.11%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main      #90      +/-   ##
==========================================
+ Coverage   78.37%   78.48%   +0.11%     
==========================================
  Files         118      118              
  Lines        8074     8134      +60     
==========================================
+ Hits         6328     6384      +56     
- Misses       1746     1750       +4     

Impacted Files	Coverage Δ
crates/ditto-ast/src/expression.rs	78.94% <100.00%> (-7.26%)	⬇️
crates/ditto-checker/src/typechecker/mod.rs	96.77% <100.00%> (+0.34%)	⬆️
...ates/ditto-checker/src/typechecker/substitution.rs	97.97% <100.00%> (+0.08%)	⬆️

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