Unlike C++, Rust requires "char" to be a valid Unicode code point. As a
workaround, we take the raw (probably UTF-8-encoded) input and convert each
input byte to a char representation from the private use area (see commit
3b15e995e (str2wcs: encode invalid Unicode characters in the private use
area, 2023-04-01)). We convert back whenever we output the string, which
is correct as long as the encoding didn't change since the data was input.
We also need to convert keyboard input; do that.
Quick testing shows that our reader drops PUA characters. Since this patch
converts both invalid Unicode input as well as PUA input into a safe PUA
representation, there's no longer a reason to not add PUA characters to
the commandline, so let's do that to restore traditional behavior.
Render them as � (REPLACEMENT CHARACTER); unfortunately we show one per
input byte instead of one per code point. To fix this we probably need our
own char type.
While at it, remove some special cases that try to prevent insertion of
control characters. I don't think they are necessary. Could be wrong..
