Remove the Unicode non-character constraint

[ndw]

I had occasion to look at the RFC 8259 grammar for JSON, wherein we find:

 unescaped = %x20-21 / %x23-5B / %x5D-10FFFF

which I’d be tempted to represent in iXML as

 unescaped = #20-#21 | #23-#5B | #5D-#10FFFF

except…#10FFFF isn’t a valid hex reference in Invisible XML because it’s a Unicode “non-character”.

I think we should probably remove the constraint on non-characters.

[ndw]

At the meeting of 3 October 2023, the CG agreed to close this issue without action.

If the input is a sequence of Unicode characters, it's not necessary to support non-characters. The RFC 8259 grammar is a leaky abstraction and we don't need to make our abstractions leaky as well.