Simplified cart predicate language

The cart predicate language has been simplified.

• [API] A boolean value is now a valid predicate. For example if a cart discount should match all carts, the predicate true is now sufficient. Previously, one had to write 1 = 1 or true = true.
• [API] A boolean function is now a valid predicate. For example lineItemExists(...) is now sufficient. Previously, one had to write lineItemExists(...) = true.