Use WeakMap to cache EnumWrapper instances

[UselessPickles]

Use a WeakMap instead of a Map to cache EnumWrapper instances. This ensures that EnumWrapper instances are garbage collected whenever their corresponding "enum-like" objects are garbage collected (no memory leaks), and should also improve cache lookup performance.

Because WeakMap solves the concern about memory leaks when ts-enum-util is used on temporary enum-like objects, there's really no need to opt out of caching any more. So the following API changes were made:

• Removed EnumWrapper.createUncachedInstance()
• Removed optional useCache param to the $enum() function (it now always uses the cache).

The $enum function was also slightly optimized by making it a direct reference to EnumWrapper.getCachedInstance.

[coveralls]

Pull Request Test Coverage Report for Build 112

• 5 of 5 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 100.0%

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Totals
Change from base Build 108:	0.0%
Covered Lines:	111
Relevant Lines:	111

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💛 - Coveralls