This gives a 25% speed boost for conversion operations on long strings
(~10,000 codepoints). For shorter strings, the speed boost is less
(as the input gets smaller, it is progressively swamped more and more
by the overhead of entering and exiting the conversion function).
When benchmarking string conversion speed, we are measuring not only
the speed of the decoder, but also the time which it takes to re-encode
the string in another encoding like UTF-8 or UTF-16. So the performance
increase for functions which only need to decode but not re-encode the
input string will be much more than 25%.
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