A version of UTF8ArrayToString that takes a length instead of a null-terminated string

[haukex]

I was in a situation where I needed to be able to get UTF-8 strings with a known length and embedded NUL bytes from C. Since it didn't seem to be supported by the Emscripten libs, and I didn't want to patch them at the time, I wrote my own version of UTF8ArrayToString - but perhaps this can be integrated.

The code is at https://github.com/haukex/emperl5/blob/emperl_v5.28.0/common_preamble.js and the use is shown in https://github.com/haukex/emperl5/blob/emperl_v5.28.0/ext/WebPerl/WebPerl.xs#L42

This seems to be related to a suggestion in #4255 (and is perhaps related to #4693).

[stale]

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