Refactor AbstractMessage.type_constraints

[treiher]

More than once I wondered what the meaning of the parameter of the type_constraints method is ("Why do I need to pass an expression to get all type constraints of a message?"):

    def type_constraints(self, expression: expr.Expr) -> Sequence[expr.Expr]:

The method should be refactored and/or documented to make their behavior more obvious.

[treiher]

Fixed in 7e0a00a.