add default Clone impl for all copyable types

[thestinger]

This provides a default implementation of Clone for all copyable types, which allows generic code to start using Clone instead of Copy.

I don't think there will be a need to expose Copy outside of this module once stuff is migrated.

[thestinger]

The test_str test case currently causes a warning - any idea how to fix this?

warning: instantiating copy type parameter with a not implicitly copyable type

[catamorphism]

The warning for that is strange -- I'll look at this more closely.

[catamorphism]

Oh, this is interesting. I think what's going on is that your default implementation of Clone applies to both &str and ~str. For no particular reason, the typechecker is picking the &str impl first, because ~str can be autosliced to &str. That causes the warning, since the autoslicing requires an implicit copy.

I haven't proved this to myself yet, but the error does go away if I replace your generic implementation with specific ones for int and ~str.

[catamorphism]

Actually, no, my previous comment is wrong. ~str isn't implicitly copyable, and only implicitly copyable types can instantiate T right now when there's a T: Copy. I'll change the test to be for @str instead.

[thestinger]

@catamorphism: hmm, would be nice to be able to have this working on non-implicitly-copyable types though

[catamorphism]

Sorry, I can't accept this as-is -- it breaks tests in std::sync (among other places) because there are existing impls of Clone (if you run make check you'll see where it goes wrong -- in rustdoc first, but that's easy to fix). That may be fixable, but it seems like this default impl has the potential to lead to overlapping instance city. I think this needs a little more thought.