This decoding required high lookahead, and was not even
consistently useful (the `garbage_char` test case was not
valid UTF-8 and so did not benefit from `reportGarbageChar`).
This limits UTF-8 handling to the `STRLEN` and `STRSUB`
built-in functions, and to charmap conversion.
POSIX leaves undefined the order of output if stderr is injected into stdout,
and in practice it differs on Windows (Linux buffers both streams separately,
Windows interleaves them as they arrive without buffering).
This should help testing on other platforms
