Add new combinator for getting ALL the dependencies of a decl.

[ChrisPenner]

Overview

Share isn't properly rendering record fields at the moment: https://unisoncomputing.slack.com/archives/CLGTK464E/p1694523701223369

The reason is that Share only pulls in names for things it knows are referenced from what's being pretty-printed,
BUT there's no actual hard-link between a record and its fields, it will only be rendered as a record if all the proper field definitions exist in the pretty-printer.

Some ideas for how I can solve this:

1. Re-generate the accessor terms for the decl I'm printing using made-up names, hash them, then use those hashes to search for the real names for those accessors so I can add them to the PPE so they'll be pretty-printed
2. Generate all the types that I know the accessors would have, then see if I can use the find_type_index to search for the terms which match those types. If I find multiple matches I guess I just include them all?
3. Give up on being clever and just recursively include every name within the subnamespace which matches the current decl's name.
4. Redesign how record fields are tracked somehow, maybe add an additional index on Share for it? This would also require changes to unison because currently the accessors are just added to the file as regular terms and after that there's no easy way to figure out where they came from.

I went with 1 for this approach and it's working 👍🏼

Implementation notes

Add a new method for getting the dependencies of a Decl which includes the field accessors.
I had to add a new module to store this in parser-typechecker because we use the typechecker in generating the field accessors.

Test coverage

Tested on local enlil

Loose ends

See enlil PR.

[ChrisPenner]

Pulled this out of the DeclPrinter and generalized it so we can share.