Reject Arc.clone() syntax in favor of Arc::clone

`arc.clone()` leaves the code somewhat ambiguous to whether we're
doing an expensive deep-clone operation or if we're doing a
(relatively) cheap pointer-copy-and-atomic-increment operation.

More importantly, it leaves entirely unclear what the semantics of
the object we just created are - does updating it update the
original or leave it be?

Thus, here we require `Arc::clone` via clippy enforcement in CI

Author: TheBlueMatt

ci/check-lint.sh

@@ -2,6 +2,8 @@
 set -e
 set -x
 RUSTFLAGS='-D warnings' cargo clippy -- \
+	`# Things clippy defaults to allowing but we should avoid` \
+	-D clippy::clone_on_ref_ptr \
 	`# Things where clippy is just wrong` \
 	-A clippy::unwrap-or-default \
 	`# Errors` \