Refactor ClientCirc to be explicitly Arc.

As discussed in Costa Rica:

In order to get reasonable behavior from our RPC engine, we change many of our types that are currently "Implicitly Arc" to instead have an explicit Arc form. The one where this is most key is ClientCirc.

(The alternative would be to define a WeakClientCirc type, and a generic WeakObjRef trait that it would implement, and probably more machinery that just duplicates Arc/Weak without reason.)

This represents a pretty big API break, so we may as well do it soon.