Declaration AST placement

[maximiliankaul]

This is just a reminder to a discussion between @oxisto and myself: 9d02f6c

[oxisto]

Currently, we place a declaration both on the AST property declarations directly as well as indirectly through the statements property and a DeclarationStatement. This could potentially be a bad idea, since the AST should be a tree, not a graph.

[konradweiss]

Yes, the ast should always be a tree. If it has more connections this can break several things, in the worst case lead to cycles in some algorithms

[oxisto]

Also for this discussion: Currently we model a a: Int or a simple a (the statement, not the expression) as an assignment with no rhs. This works sort-of and the assignment expression will also add a declaration, however the question would be whether we want to directly translate this into a declaration statement / variable declaration.