arbitrary v1.2.0 + derive doesn't build with generate-lockfile -Z minimal-versions

[tarcieri]

It pulls in derive_arbitrary v1.1.6 which has the following build failure:

error[E0599]: no method named `len` found for reference `&Fields` in the current scope
   --> /home/runner/.cargo/registry/src/jackfan.us.kg-1ecc6299db9ec823/derive_arbitrary-1.1.6/src/lib.rs:219:30
    |
219 |         if idx + 1 == fields.len() {
    |                              ^^^ method not found in `&Fields`

Perhaps you could bump the derive_arbitrary requirement in arbitrary to 1.2.0 (it's presently 1.1.6) or yank the affected derive_arbitrary v1.1.6 release?

[fitzgen]

@Manishearth FWIW, this kind of thing is the reason why I try to always keep the versions in lock step with each other.

[Manishearth]

Yeah, fair, I still think it should be fine for patches.