module to add line numbers to eval'ed heredoc blocks

 Eval::LineNumbers adds a '#line "this-file" 392' comment to heredoc/hereis
 text that is going to be eval'ed so that error messages will point back to
 the right place.
 .
 Please note: when you embed \n in your code, it gets expanded in double-quote
 hereis documents so it will mess up your line numbering. Use \\n instead when
 you can.