Add (T, U, V) triple primary key

[ethanfrey]

This makes very clear the distinction between prefix and suffix.

We should be able to range by fixing (T, U) and iterating over V

Let's test both the following: ((T, U), V) and (T, U, V). For both, we should be able to give (T, U) and iterate over V.

Example: (&[u8], U32Key, U64Key)