`Traverse` use `Semantic` to construct scopes tree and expose it to visitors via `TraverseCtx`.
Currently scopes tree is immutable. Will expose it as a mutable in a follow-on.
This is extremely inefficient. Semantic does all kinds of stuff (control flow graph etc) which `Traverse` doesn't need, and `Traverse` just throws away all that work after semantic has done it. Intent here is to get a working implementation first, and then to do another pass later on to improve performance.
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