Move plugin helpers out of Parser.prototype and into the plugin itself

[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

Plugins used to define helper functions and overrides separately, likely to avoid adding too many instance fields to the parser object. Now that each plugin is a class (#459), that is no longer necessary.

I tested with the perf script and this doesn't seem to have any impact.

[codecov]

Codecov Report

Merging #482 into master will decrease coverage by <.01%.
The diff coverage is 99.13%.

@@            Coverage Diff             @@
##           master     #482      +/-   ##
==========================================
- Coverage   98.24%   98.24%   -0.01%     
==========================================
  Files          20       20              
  Lines        3531     3528       -3     
  Branches      973      973              
==========================================
- Hits         3469     3466       -3     
  Misses         23       23              
  Partials       39       39

Flag	Coverage Δ
#babel	81% <76.79%> (-0.22%)	⬇️
#babylon	97.08% <99.13%> (-0.01%)	⬇️

Impacted Files	Coverage Δ
src/plugins/estree.js	99.16% <100%> (-0.01%)	⬇️
src/plugins/jsx/index.js	97.7% <97.01%> (-0.01%)	⬇️
src/plugins/flow.js	98.15% <99.82%> (-0.01%)	⬇️

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