Speedup of functions to en/decode UTF-8 text

[Eli Zaretskii]

I've made some changes in the functions that encode and decode UTF-8
text in an optimized fashion.  This is expected to speed up JSON
handling.  Some preliminary benchmarking of the decoding function
indicates speed improvements starting at modest 30-40%, and sometimes
much higher -- tenfold or more.  I'm not sure how this will fare in
the real-life handling of JSON objects, so I'd welcome speed
measurements of the current master branch as compared to previous
versions (yesterday and older) in real-life scenarios, in particular
in LSP and Eglot.  Please report the results here, and if there are
some significantly adverse effects of these changes, please report
them as bugs.

TIA