This patch provides a semantically neutral fast-path for cases when
the path does not need to be encoded (up to 5x speedup), reduces
allocation when the string has a prefix that does not need to be
encoded (1-2x speedup) and no regression when using a separator
that's not '/' or the first char needs encoding.
Interpreted performance is not affected much either: small positive
when no encoding is needed, neutral or negligible regression