900: Add new trait ast::TypeAscriptionOwner r=vipentti a=vipentti
This trait should be implemented for nodes which have an ascribed type,
e.g. thing : Type. Such as let, const, static, param, named struct fields.
In addition, we update some places where previously we used node + node.type_ref() with `TypeAscriptionOwner` in the trait bounds.
Co-authored-by: Ville Penttinen <villem.penttinen@gmail.com>
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