Detangling Driver and DriverMode

[Ekleog]

So, continuing the discussion started on https://github.com/camshaft/bolero/pull/108#discussion_r1083565909 ; let's chat about driver and driver mode here.

I guess my first question would be, is there any user-facing use case for something other than (DriverMode::Forced, Driver::Rng) (for proptesting) and (DriverMode::Direct, Driver::ByteSlice) (for fuzzers)? And is there a non-user-facing use case for anything other than (DriverMode::Forced, Driver::ByteSlice), that's I guess used for input shrinking when proptesting?

In particular, do you know why DirectRng is being exposed?

Also, writing down my current plan for changes of *Generator while I have it in mind: instead of fn(Driver) -> Generated, have it be fn(Driver, DriverMode, depth: usize) -> Generated. Use the depth to generate smaller collections when going deeper in Forced mode, and to refuse generating too deep data structures like arbitrary in Direct mode.