Negative array index is ~25% faster than Array#last.
Move finish method after append.
Prioritize quote comparison over closing brace.

In real documents, this avoids hundreds of extra String#=== comparison
calls.
Prioritize closing object brace over array bracket.

It’s more common to have arrays of objects than nested arrays, so this
avoids extra String#=== comparison calls.
Skip buffer if data is already well-formed UTF-8.

Provides ~20% performance increase.