remove 78-level constants from the lexer

[lefessan]

To get better error messages.

If I understand well, the idea is to remove 78-level constants from the lexer, and replace all the places where they can replace a LITERAL by a rule that also accepts a WORD and checks if it is a 78-level ?

Originally posted by @lefessan in #133 (comment)

[GitMensch]

Not completely, the lexer would return a new token (which gets a new tree if CB_CONST is not good enough), so the parser knows that this is a constant.

Note: This should be handled identical for both the extension level 78 constant and the standard 01 CONSTANT AS.