Refine conversions between array types

[jturner314]

• Move into_shared from DataOwned to Data, add an A: Clone bound, and drop the "without any copying" guarantee. (See the discussion in #1021.)

• Implement From for more combinations of array types.

[bluss]

I'm not sure about going from DataOwned to Data. I guess there are many alternatives for how it could work. I was first thinking of - unsure if we can realize it - a conversion of owned arrays only, which would move if needed otherwise clone.

[jturner314]

Okay, yeah, leaving it on DataOwned would be fine. We'd still add an A: Clone bound, and drop the "without any copying" guarantee. OwnedRepr, OwnedArcRepr, and the Owned variant of CowRepr would implement this without copying, but the View variant of CowRepr would copy.