Enable clippy rule clone_on_ref_ptr

[akoshelev]

It is disabled by default.
https://rust-lang.github.io/rust-clippy/master/#clone_on_ref_ptr
Motivated by rust-lang/rfcs#2588

[martinthomson]

Given that CI already runs with clippy::pedantic, isn't this covered already? We just have memorialized it in the code, but we could do that instead. (Adding one lint at a time seems like it might take a lot of effort.)

[akoshelev]

Given that CI already runs with clippy::pedantic, isn't this covered already? We just have memorialized it in the code, but we could do that instead. (Adding one lint at a time seems like it might take a lot of effort.)

I think the default clippy setting is allow, so clippy::pedantic does not block these. I thought we could use .clippy.toml for it but it is marked unstable so given that we don't have code that uses <arc>.clone() yet, this might be a good way to prevent it from appearing in the repository. I think the pipeline PR violates this rule already

[martinthomson]

Oh, I didn't check the rule's default enforcement. I wonder why it never made it into the pedantic set...

If we merge this first, the pipeline code will hit it :)

[akoshelev]

I think it is considered controversial rust-lang/rust-clippy#2048, but we can see whether it works for us :)