Fixing an issue with Tokens creating ambiguities with variables.

@andreistefanescu requested this fix.
Now, if you write Id ::= "Main", whenever you will use the token 'Main' in rules or programs, it will be transformed into a token.
The general rule is: if you declare Token{...}, that sort will become a lexer sort. For lexer sorts, if you declare another production with a single terminal, that production will produce a generic token instead of a klabel.