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:

1. 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.

2. 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.