Change AST representation of emit statement

[cburgdorf]

What is wrong?

The emit statement is currently represented in the AST as:

Emit {
        value: Node<Expr>,
    },

This suggests that the right hand side can be any expression such as emit get_some_event() or emit SomeEvent() if condition else OtherEvent().

However, that is not actually allowed and we reject these cases in the analyzer pass. If the right hand side is not actually allowed to be an expression (it is unclear to me if it ever could) then we should change the AST representation to something that prevents that in the first place.

How can it be fixed

1. Probably change the Emit node to something with just name and args fields
2. Change the parser to parse it according to the new representation
3. Refactor the analyzer pass to get rid of the current checks that are concerned with syntax

[cburgdorf]

@sbillig just to let you know, I started working on this today to get familiar with the new parser. I should have something to review for you soon :)