This PR changes the rule for which projections become instances. Before,
all parents along with all indirect ancestors that were represented as
subobject fields would have their projections become instances. Now only
projections for direct parents become instances.
Features:
- Only parents that are not ancestors of other parents get instances.
This allows "discretionary" indirect parents to be inserted for the
purpose of computing strict resolution orders when
`structure.strictResolutionOrder` is enabled, without having an impact
on typeclass synthesis.
- Non-subobject projections are now theorems if the parent is a
proposition. These are also no longer `@[reducible]`.
Closes#2905
