EXP: try out lower level pyo3 API

[ctb]

Copy over useful code from #428, to try anew.

---

This PR -

• provides an initial wrapping of Collection, Manifest, and Record;
• adds a load_from hook to support loading files via new wrapping;

Next steps:

• adjust Record to provide getters for private elements ksize, scaled, num, with_abundance, and n_hashes;
• think about how to support more of the Index protocol - in particular, providing sketches that are compatible with the FFI layer, and supporting find, prefetch, and peek;

Other notes:

pyo3-ffi - might this let us work directly with FFI interface in sourmash?

Limit to rocksdb loading? And/or provide env variable switch to turn loading on and off?