adjust JSON signature format to better support streaming and progressive I/O?

[ctb]

maybe we should adjust JSON signature format to support a collection of individual records, rather than a list?

Right now, the JSON output by sourmash is a list of signature records. There's no reason we couldn't also support multiple individual records, is there?

from #1440 (comment),

reference to explore streaming JSON output - https://medium.com/galvanize/streaming-structured-json-18da4edd4f20

Note: incorporating suggestion from #1093,

right now to combine signatures from multiple files into one, you need to use sourmash sig cat - it'd be nice to permit just cat.

this issue would address that, too, I think.

[ctb]

per @luizirber on slack -

good future project: turns out it is not so hard to make a streaming
parser out of serde, so we can load partial data (like specific ksize, or
downsampling while reading the data): https://serde.rs/stream-array.html

[ctb]

note from #1097 (comment) -

it seems also like zip files are a better answer to this.

[ctb]

basically it's not clear this is worthwhile, given our other signature save formats (.zip etc). We should just be using direct random-access signature formats, as suggested in #1262. Closing.