haddock: non-ASCII characters in comments

[Ross Paterson]

It would be useful to have a portable way to markup non-ASCII characters
in comments.  Some possibilities:

	&#xhhhh; or &#dddd; (as in SGML)
	\xhhhh (possibly with \&, as in Haskell strings)
	\uhhhh (as formerly in the Haskell Report)

These could be converted to Char by the lexer, so there would be no
direct connection to the output format (the HTML backend renders them
as &#xhhhh;).