Scanner Generation - Downcasting from int to char

[Ynscription]

The generated function of the Scanner AddCh() downcasts the int ch where the current character is stored with UTF-32 encoding to a 16 bit char that expects UTF-16 encoding.

This means that glyphs that require surrogate UTF-16 pairs are not being scanned corectly.

[robin-aws]

Closely related second issue to this: Buffer.EOF is defined to be char.MaxValue + 1, or 0x10000. This is a valid Unicode code point, so if it appears in the input the parser will get confused and terminate early, or hit IndexOutOfRangeExceptions.

I'm working around both of these issues in this Dafny PR: dafny-lang/dafny#3016. It would be great to have Coco itself fixed instead though - feel free to steal my fixes there if I don't come back here with a PR in the near future. :)