Make AST more ambiguous

[bmsherman]

When a user types a + in their Cypher syntax, there's a good chance that we won't be able to know whether they mean to add numbers or concatenate strings. So instead of having EPlus :: E Number -> E Number and EConcat :: E String -> E String -> E String, we need to have something like EPlus :: Addable a => E a -> E a -> E a