pkg/types/camtypes: use unique.Handle

// BlobMeta is the metadata kept for each known blob in the in-memory
// search index. It's kept as small as possible to save memory.
type BlobMeta struct {
	Ref  blob.Ref
	Size uint32

	// CamliType is non-empty if this blob is a Perkeep JSON
	// schema blob. If so, this is its "camliType" attribute.
	CamliType schema.CamliType

	// TODO(bradfitz): change CamliTypethis *string to save 8 bytes
}