Outer parser - improving lexical expressions and adding case insensitive keywords

@dwightguth please review.
 - case insensitive Terminals. Just like strings but with single quote
 - fixing and improving lexical expressions to match SDF. Allowing users to reference other lexical expressions.
 - improved the error message when having an ambiguity between two constants.

Calculating the automatic token rejection, now has to take into consideration the two types of strings, so I'm using Terminals directly.