Remove IntoOwned

[frankmcsherry]

This PR moves the functionality of IntoOwned into the types that first provide access to the owned and borrowed types. The goal is to remove the freestanding requirement that a reference type should implement IntoOwned, as this is generally hard to introduce if it does not already exist. On the other hand, it is relatively easy for a container to provide this functionality, as it only needs to wrap existing logic (assuming it exists; if not there is a different problem). The main distinction is that we are happy to create new containers, or wrapper containers, etc. but much less happy to create new types that they contain.

There is a surprising amount of unmucking to do along the way, and this description will be updated with the narrative arcs if it is successful. For the moment, it is a draft PR revealing the work.

[antiguru]

I think this looks good. Left some comments, but let's merge soon.

[frankmcsherry]

Thanks for the review, and agree about wanting to land this where possible. I'm pretty confident that the path ends up in the intended state, and checkins are perhaps locally clarifying even if the final arc of the store isn't yet clear. :D