A protocol that's been reparented declares it
by writing `@reparented` in its inheirtance clause
for each new parent. You can introduce a `@reparented`
parent to a pre-existing ABI-stable protocol's
inheritance hierarchy.
Only protocols declared to be `@reparentable` can be
used to reparent other protocols. Adding or removing
the `@reparentable` attribute is ABI-breaking, as it
effects the type metadata layout. Thus, reparentable
protocols must be born as such to use them with
protocols that are already ABI-stable.
This set of changes does not include the actual
implementation of ABI-stable reparenting.
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