Type-check StatementParser

[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.
Also switched ExportNamedDeclaration.specifiers to use a $ReadOnlyArray.

[codecov]

Codecov Report

Merging #489 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #489      +/-   ##
==========================================
+ Coverage   98.26%   98.27%   +<.01%     
==========================================
  Files          22       22              
  Lines        3526     3527       +1     
  Branches      978      978              
==========================================
+ Hits         3465     3466       +1     
  Misses         22       22              
  Partials       39       39

Flag	Coverage Δ
#babel	81.08% <95.52%> (ø)	⬆️
#babylon	97.1% <100%> (ø)	⬆️

Impacted Files	Coverage Δ
src/parser/statement.js	99.08% <100%> (ø)	⬆️

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