Type-check LValParser

[ghost]

Q	A
Bug fix?	no
Breaking change?	no
New feature?	no
Deprecations?	no
Spec compliancy?	no
Tests added/pass?	Tests pass
Fixed tickets	comma-separated list of tickets fixed by the PR, if any
License	MIT

Enables type-checking in the file and annotates every method.
Requires a few forward-declarations for expression.js methods. In a few cases these could be moved to a higher class because they aren't really expression-specific (checkReservedWord, parseIdentifier, parseDecorator), but trying to keep this pull request to just type declarations.
Remove unnecessary (foo : Array<Object>) casts.

[codecov]

Codecov Report

Merging #487 into master will not change coverage.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master    #487   +/-   ##
======================================
  Coverage    98.2%   98.2%           
======================================
  Files          21      21           
  Lines        3519    3519           
  Branches      974     974           
======================================
  Hits         3456    3456           
  Misses         24      24           
  Partials       39      39

Flag	Coverage Δ
#babel	80.9% <92.3%> (ø)	⬆️
#babylon	97.04% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/parser/lval.js	97.94% <100%> (ø)	⬆️

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