Data interoperability between other MinHash implementations

[edawson]

As noted by Titus in an issue, Minhash sketches should be stable and cross-compatible between programs. It makes sense to fall into line with this convention. To this end, rkmh should output either Mash's capnproto schema or a text file akin to sourmash's YAML sigs.

[edawson]

Commit b9224 partially solves this, but there are still a few items left:

1. JSON output is ordered alphabetically, which may or may not be okay.
2. There is no way (at least from the CLI) to change hash seeds, hash functions, alphabet, whether to use non-canonical bases, or the number of hash bits.
3. Reads/Refs are spat out individually at the moment, which is different than Mash/Sourmash, which collate sets of reads/refs into sketches. I guess there should be a switch for this a la mash -i?
4. There is no error handling in rkmh hash yet, and there're probably still some performance issues. I have no idea if the structures json.hpp provides are thread-safe, nor have I tested with multiple threads.
5. There's no way to read the sketches back in, so they're essentially useless at the moment (outside of for debugging purposes).