refactor: Don't concatenate strings

[fb55]

Refactors tokenizer callbacks to pass indices instead of strings. This allows us to avoid most string concatenations, which will speed things up in production.

This should also help with memory bloat: V8 doesn't garbage collect strings with slices still being referenced.

Note: AndreasMadsen/htmlparser-benchmark only parses whole documents. As a consequence, there is no speedup with this change.

[coveralls]

Pull Request Test Coverage Report for Build 1580285346

• 328 of 331 (99.09%) changed or added relevant lines in 2 files are covered.
• 1 unchanged line in 1 file lost coverage.
• Overall coverage decreased (-0.5%) to 98.991%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
src/Tokenizer.ts	238	241	98.76%

Files with Coverage Reduction	New Missed Lines	%
src/Tokenizer.ts	1	98.75%

Totals
Change from base Build 1580203836:	-0.5%
Covered Lines:	785
Relevant Lines:	793

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💛 - Coveralls