Remove more probe-specific functions from Core

[mkeeter]

This PR removes reset, reset_and_halt, and wait_for_halt from the Core trait. These functions are only implemented on ProbeCore (and UnattachedCore::reset), so removing them from the trait lets us be more precise with our APIs.

(wait_for_halt is not used anywhere, so it's been fully deleted).

The only tricky part is AuxFlashHandler, which only sometimes needs a full ProbeCore – basically only when writing. I added a new struct AuxFlashWriter, then an AuxFlashWorker trait to unify common functions (e.g. getting slot count, and active slot). This is a bit annoying, but minimizes boilerplate; I couldn't find a way to write a type which accepts either a &mut dyn Core or a concrete C: Core without replumbing everything.