propose updates to sourmash to make kmers_and_hashes code less cumbersome

[ctb]

code in #40 could benefit from making SeqToHashes less opaque over in sourmash.

Some ideas:

• simple and lightweight: provide access to kmer_index per MRG: add kmers_and_hashes method to get canonical kmers + hashes #40 (comment) to make
• better, but more: provide an iterator in sourmash that yields canonical k-mers and hashes;

[ctb]

ref sourmash-bio/sourmash#3339