Rename some internals for clarity

[Zac-HD]

Naming things is famously, uh, less easy than we might hope. This issue tracks two renames I'd like to do at some point:

• Rename the conjecture.data.Example class to Span, as suggested in Experimental changes for Crosshair support #4164. The word "example" is pretty badly overloaded in our codebase, and so using a short and distinct word should help - I think it's also more indicative of what this subsystem does. We'll wait for the IR changes to settle down first though.

• Rename the "primary" and "secondary" database keys to indicate that they store minimal and non-minimal failing examples respectively. This has caused confusion before, see e.g. Add fast path for replaying already shrunk test cases #4155, and should smooth out interoperability with e.g. HypoFuzz. I'm inclined to come up with canonical names + keys for covering examples (including un/shrunk, and location-vs-set-of-locations), and likely provide a helper function to compute said keys.