implement shallow_clone for RObject/DynTrait

[RustyYato]

Fixes #107

Implement a shallow_clone function on RObject/DynTrait to allow cloning trait objects without cloning the underlying value.

[RustyYato]

I opted for shallow_clone instead of cloned. Because in std, cloned is only used when you have a transformation like T<&U> -> T<U> where U: Clone. Which is a different operation.
I can't use clone since that would be a breaking change, so I picked a new name shallow_clone, which doesn't have some existing usage in std but better describes what's going on.

[rodrimati1992]

Looks alright.

[rodrimati1992]

Here's hoping that the the ICE that affects abi_stable gets fixed in the Rust beta channel that GitHub uses soon, because it's making more complex PRs unmergeable until it's fixed.

[RustyYato]

Yeah, I'm pinned to a June nightly to work around the ICE, hopefully, that gets fixed for beta!