Housekeeping: let work tell us dependencies and what it reads/writes

[rsheeter]

Large but mostly boring cleanup. Took longer than I care to admit to get everything wired up and working again :D

Move the knowledge of read/write access to the work. Define dependencies in terms of read access: if you can read it you depend on it and therefore must happen after it. Eliminate separate storage of dependencies, it was duplicative and quite weird when inconsistent with read_access.

Eliminate the macro for Context items, preferring to instead write reusable types ContextItem and ContextMap, as @cmyr once suggested.

Files (there are several) named orchestration.rs are probably the most interesting part; the rest is essentially repetitive updates to match the new interfaces. I would start with fontdrasil/src/orchestration.rs, then fontir/src/orchestration.rs and finally fontbe/src/orchestration.rs.

[cmyr]

Did a quick read through the various orchestration files, and have a few notes inline; overall this is a nice improvement, and it's nice to see you being more comfortable mucking about with the trait system.

[cmyr]

A few more little non-blocking things, but overall this feels like a big improvement. I'm happy to see this get merged, and then we can iterate from here.