Type-check ExpressionParser

[ghost]

Q	A
Bug fix?	no
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	no
Tests added/pass?	Tests pass
Fixed tickets
License	MIT

Enables type-checking in the file and annotates all methods.
Requires a few forward-declarations for statement.js. parseClass might deserve its own file; same for decorator-related methods. But trying to just do type annotations in this PR.

[codecov]

Codecov Report

Merging #488 into master will decrease coverage by <.01%.
The diff coverage is 97.87%.

@@            Coverage Diff             @@
##           master     #488      +/-   ##
==========================================
- Coverage   98.26%   98.26%   -0.01%     
==========================================
  Files          22       22              
  Lines        3526     3525       -1     
  Branches      978      978              
==========================================
- Hits         3465     3464       -1     
  Misses         22       22              
  Partials       39       39

Flag	Coverage Δ
#babel	81.07% <95.74%> (-0.01%)	⬇️
#babylon	97.1% <97.87%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/parser/expression.js	97.38% <97.87%> (-0.01%)	⬇️

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[ghost]

Codecov is having problems here; don't know where the coverage was lost.