add collection filtering to constellation for jetstream · Pull Request #55 · microcosm.blue/microcosm-rs

hello! sorry i've been heads-down and not checking tangled. this is great! long-time todo, thank you.

a few notes:

there are two collections involved with every link: the collection of the linking record, and the linked-to record. we don't need to add both right away, but i think the param name and its doc should reflect that this filters for "links from" these collections.
lil query injection risk the querystring builder. no security risk, and i believe NSIDs disallow any characters that could break anything (no & etc)... eh i'm already talking myself out of it. if there was an Nsid type to grab already in this project i'd say we should use that instead of String, but this is fine.

printing the filter on startup is appreciated!