Removed literals from parseExpression, pointed it to parseLiteralValues

Also removed parseAtom from parseLiteralValues, since an atom is a
pointer to something, not a literal value.
Replaced use of parseLiteralValues->parseExpression in parseDeclaration

So now we can do:
Dim x = System.String.Format("...{0}...", 12)

Of course the evaluation portion of the code doesn't know how to handle
dot notation or method calls yet.