Use our own escape character parser.
We're currently using the spirit escaped character parser. This is a
strict C parser and causes parse errors for code that we probably should
accept, such as any hexadecimal encoded characters with a value greater
than the maximum value of the current platform's 'char'. This is \x7F
on some platforms so it rejects '\x80' upwards. As well as rejecting
characters that might be valid, it also means that quickbook will act
differently on different platforms.
In python strings, '\xaaa' is equivalent to '\xaa' + 'a', but the spirit
parser interprets this as a character with value '0xaaa'. So we probably
should accept these.
I also think we should be liberal about what we accept. IMO it isn't our
job to enforce correct C++/python, just to create a reasonable
rendering of our input. So rather than write a parser which understands
the various types of escapes, I just wrote one that ignores any
character following a backslash.
Refs #2860.
