As with the JSON writer, the old code was made to be simple and
for each char just decided whether it needs escaping, or should be
written as-is. The new code instead looks for characters that need
escaping and batches writes of characters that do not.
This provides 4-8x speedup (length dependent) for writing strings
that do not need escaping, and keeps roughly the same performance
for those that do need escaping.
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