Consolidate helper code into its own file

[alangpierce]

Now that we have four helpers and will soon be adding a fifth, it's nice to not
need a variable and lazy-getter for each one, so this puts them all in a file
that defines all helpers and coordinates those details.

This might be too dynamic to work in AssemblyScript, in which case I'd switch it
to use a codegen approach instead.

[codecov-io]

Codecov Report

Merging #419 into master will increase coverage by 0.03%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #419      +/-   ##
==========================================
+ Coverage   80.05%   80.09%   +0.03%     
==========================================
  Files          46       47       +1     
  Lines        5040     5264     +224     
  Branches     1180     1174       -6     
==========================================
+ Hits         4035     4216     +181     
- Misses        702      744      +42     
- Partials      303      304       +1

Impacted Files	Coverage Δ
src/CJSImportProcessor.ts	91.66% <100%> (-0.3%)	⬇️
src/HelperManager.ts	100% <100%> (ø)
src/util/formatTokens.ts	70.58% <0%> (-10.06%)	⬇️
src/parser/util/identifier.ts	90.9% <0%> (-4.1%)	⬇️
src/parser/plugins/jsx/index.ts	87.33% <0%> (-1.88%)	⬇️
src/parser/plugins/flow.ts	62.21% <0%> (-1.59%)	⬇️
src/util/getClassInfo.ts	85.12% <0%> (-1.21%)	⬇️
src/identifyShadowedGlobals.ts	87.8% <0%> (-1.09%)	⬇️
src/parser/traverser/statement.ts	80.46% <0%> (-0.78%)	⬇️
src/util/isIdentifier.ts	100% <0%> (ø)	⬆️
... and 17 more

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