Allow compilation with `--cfg fuzzing` but not `--features=arbitrary`

[smoelius]

Right now, if quinn-proto is built without --features=arbitrary, one gets an error about an unresolved import. The compile_error helps to clarify why.

An alternative would be to try and disentangle --cfg fuzzing and --features=arbitrary. But this change is less invasive.

[djc]

An alternative would be to try and disentangle --cfg fuzzing and --features=arbitrary. But this change is less invasive.

Did you try looking at this? I think I would usually prefer disentangling over this.

[smoelius]

Did you try looking at this? I think I would usually prefer disentangling over this.

I did not, but if you hold off on merging, I will.

[smoelius]

@djc Is this better?

[djc]

Yes, thank you!