Add `DefId::parent()` accessor for `rustc_public`
Adds a `parent()` method to `DefId` (the `rustc_pub` version) which exposes the parent path, ie. `foo::bar::baz` -> `foo::bar`.
This is useful for organizing/grouping definitions into a tree, and is probably simpler and less brittle than attempting to parse the fully-qualified name into path components (e.g. especially when handling path components with qualified generic parameters).
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