remove the copy keyword, make Copy represent implicitly copyable types

[thestinger]

I think this is the consensus after quite a few discussions about this. Clone will be the way to explicitly copy an ownership tree (stops at @ boundaries).

In the future, a DeepClone trait could be implemented to copy past @ boundaries, but it will have to handle cycles to work in the general case.

[graydon]

Still legit, properly classified, not yet done. 2013-06-19.

[thestinger]

The copy keyword is now gone, and Copy has been removed. Adding a trait for implicitly copyable types can be a separate issue not tied to backwards compatibility.