implicit 'it' parameter created wrongly

The closures {:: y} and

have different semantics, but the present system treats them the same, giving both an implicit parameter list of (it = null). A closure with an explicit empty parameter list should not get it changed to another parameter list, because the user has explicitly specified one.

Suggested fix: Move the logic to create the (it = null) list into the Antlr parser plugin, and respect the different between PARAMETERS and IMPLICIT_PARAMETERS at that transformation point.