Extend Fuzzer to Check Debug Locations

[d-sonuga]

For #194. The checker can now check whether the entries in the debug_locations output contain the vregs they're expected to contain as indicated by Function::debug_value_labels. The checking doesn't happen by default; instead, it's only enabled during fuzzing.

[d-sonuga]

@cfallin, I'm definitely aware that this is inefficient: I actually just wanted some thoughts on whether or not the correctness of the debug locations is actually being checked reasonably.

My bad for not being explicit about that in the initial comment :sweat_smile:.

[cfallin]

@d-sonuga ah, in that case, yes the checks do look correct at least. I'm happy to leave this open as a draft if you'd like or we can close it, up to you.

[d-sonuga]

I'd prefer to leave it open as a draft. I still intend to work on this.