Reverse Lexer

[srhickma]

Allow the lexing direction to be specified for a CDFA region, which will determine the direction that the input is scanned. This will be particularly useful for #180. The syntax could look something like:

cdfa::forward {
 // Scan these rules forward
}

cdfa::reverse {
 // Scan these rules backward
}

cdfa {
 // Default direction is forward
}

[srhickma]

This could be implemented using #183

[srhickma]

This is no longer needed for #180, as there are simpler ways to achieve a similar lexing style, so closing for now.