Introduce a CodePointTrieBorrowed-using borrowed counterpart of CollationDataV1

[hsivonen]

CollationElements should hold the new type instead of holding references to CollationDataV1.

See #2554.

[sffc]

The approach I'd recommend here is what we've done in Segmenter and Properties. Have a "foundation" type that holds ownership of the DataPayloads, and then an intermediate type that borrows everything it needs from the foundation. This way you can pull out both the DataPayload::get as well as the ZeroSlice.