refactor: move eval into a separate primer-eval package

[dhess]

Still very much a WIP. The tests need fixing up, and primer-service will need to combine the sessions-and-actions API with the factored-out eval API. None of that work has been done yet, but primer and primer-eval do compile, at least.

I'm open to suggestions/techniques for how best to combine the two APIs.

(One could also imagine a further evolution where we move actions & type checking to the frontend, as well, leaving only the sessions API, i.e., program store, for primer-service.)

[dhess]

After chatting with @brprice today, I think this is probably the wrong approach, or at least, the wrong order in which to do things. The tentative plan now is:

1. "chop off" the Primer.API module and its dependencies into a new primer-api package.
2. Split out the eval-related bits of the remaining core package into primer-eval, similar to what I've done here.
3. Optionally split out the type checking & editing bits of the core package into, e.g., primer-typecheck, leaving primer-core as the bits that both primer-eval and primer-typecheck depend on.

Then we'd have a pretty good story for compiling primer-eval to Wasm and running that in the browser, with a minimal footprint of code.