i18n: Memoize dcnpgettext

[aduth]

This pull request seeks to memoize the result of the i18n module's central dcnpgettext function. This improves performance of a simple call to __ by roughly 26000% .

Before:

__ x 145,625 ops/sec ±1.01% (90 runs sampled)

After:

__ x 37,909,434 ops/sec ±0.58% (85 runs sampled)

The root of the performance issue is Jed's Jed.PF.compile function, which compiles a plural forms string into an AST, returning a function which interprets a count value to its plural form. There is no caching for this function, and it is invoked on every call to dcnpgettext for the default locale. The first implementation here simply memoized Jed.PF.compile, which improved performance significantly and had the advantage of likely caching just a single value function result. The main downside is that we'd be modifying the module directly, so any other dependents would inherit the same behavior. The approach here is isolated to the i18n module and is more performant, at the expense of higher memory cost of caching the result of each distinct translation call.

[codecov]

Codecov Report

Merging #101 into master will decrease coverage by 0.47%.
The diff coverage is 16.66%.

@@            Coverage Diff             @@
##           master     #101      +/-   ##
==========================================
- Coverage   64.54%   64.07%   -0.48%     
==========================================
  Files          41       42       +1     
  Lines         598      604       +6     
  Branches      118      119       +1     
==========================================
+ Hits          386      387       +1     
- Misses        170      175       +5     
  Partials       42       42

Impacted Files	Coverage Δ
packages/i18n/benchmark/index.js	0% <0%> (ø)
packages/i18n/src/index.js	95% <100%> (+0.26%)	⬆️

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