Handling invalid UTF sequences

[Walter Bright]

Currently we do it by throwing a UTFException. This has problems:

1. about anything that deals with UTF cannot be made nothrow

2. turns innocuous errors into major problems, such as DOS attack vectors
http://en.wikipedia.org/wiki/UTF-8#Invalid_byte_sequences

One option to fix this is to treat invalid sequences as:

1. the .init value (0xFF for UTF8, 0xFFFF for UTF16 and UTF32)

2. U+FFFD

I kinda like option 1.

What do you think?