Now `TypeRepr` is a final class in the AST, which is more or less just
a type with a location in code.
As the frontend does not provide a direct way to get a type from a
type representation, this information must be provided when fetching
the label of a type repr.
This meant:
* removing the type repr field from `EnumIsCaseExpr`: this is a virtual
AST node introduced in place of some kinds of `IsEpxr`. The type
repr is still available from the `ConditionalCheckedCastExpr` wrapped
by this virtual node, and we will rebuild the original `IsExpr` with
the IPA layer.
* some logic to get the type of keypath roots has been added to
`KeyPathExpr`. This was done to keep the `TypeRepr` to `Type` relation
total in the DB, but goes against the design of a dumb extractor. The
logic could be moved to QL in the future
* in the control flow library, `TypeRepr` children are now ignored. As
far as I can tell, there is no runtime evaluation going on in
`TypeRepr`s, so it does not make much sense to have control flow
through them.
