returning more & richer information from rust SeqToHash struct/iterator

[ctb]

Over in oxli-bio/oxli, @Adamtaranto and I have been implementing k-mer functionality (no sketching!) on top of SeqToHashes in Rust, and then exposing it to Python via pyo3.

Most recently, we just added a kmers_and_hashes method that returns canonical k-mers and their hashes see oxli-bio/oxli#40. We're talking there and in oxli-bio/oxli#66 about how to pull relevant functionality back into sourmash, too.

We are not yet using protein k-mers, but we hope to get there soon: oxli-bio/oxli#38

Some fun related issues popped up in this repo -

• @Adamtaranto has been exploring this for longer than I realized 😆 - How to replicate sourmash hash function #2455
• Add k-mer positions while sketching #2985 suggests supporting an iterator that yields 4-tuples: (hashval, sequence, strand, position)
• also see Return k-mers in addition to sketch/hash values #2073 where this functionality is requested as part of the sketches
• we could slay WIP: Fix sig kmers #2856 as part of this