Improve handling of qq(\N{...}); and /x

Improve handling of qq(\N{...}); and /x

It is possible to bypass the lexer's parsing of \N.  This patch causes
the regex compiler to deal with that better.  The compiler no longer
assumes that the lexer parsed the \N.  It generates an error message if
the \N isn't in a form it is expecting, and invalid hexadecimal digits
are now fatal errors, with the position of the error more clearly
marked.

The diagnostic pod has been updated to reflect the new error messages,
with some slight clarifications to the previous ones as well.