activity: moving helpers to lib

[arthyn]

OTT, makes reading through the agent much nicer

PR Checklist

• Includes changes to desk files
• Describes how you tested the PR locally (test ship vs livenet)
• If a new feature, includes automated tests
• Comments added anywhere logic may be confusing without context

[Fang-]

Nice, looks better indeed. How would you feel about further splitting the library up into scope-separated cores? ie having +get-parents:source:lib instead of +get-parents:lib, +get-reads:stream:lib, etc? (Though probably +src:lib and +stm:lib or w/e to avoid shadowing too hard.)

May or may not make sense for some of those to be written as doors, too, not sure how the ergonomics of that would shake out.

[Fang-]

As discussed out of band, thanks for your changes. Approved!