Implement a copy operator and make it mandatory in some cases

[pcwalton]

For what cases, we aren't sure at the moment.

[eholk]

If this is for deep copies, we'll probably need it for sending things to tasks.

[brson]

So we have a copy operator now. We also have alias analysis that does implicit copies. We could turn those implicit copies into errors, which is arguably more in the spirit of rust. Additionally, I'm not convinced of the need for a copy operator, since it can be simulated with { } and implemented in std.