Stabilize clone

This patch marks `clone` stable, as well as the `Clone` trait, but
leaves `clone_from` unstable. The latter will be decided by the beta.

The patch also marks most manual implementations of `Clone` as stable,
except where the APIs are otherwise deprecated or where there is
uncertainty about providing `Clone`.
diff --git a/src/libcollections/ring_buf.rs b/src/libcollections/ring_buf.rs
index cdb92d3..5d53520 100644
--- a/src/libcollections/ring_buf.rs
+++ b/src/libcollections/ring_buf.rs
@@ -48,6 +48,7 @@
     ptr: *mut T
 }
 
+#[stable]
 impl<T: Clone> Clone for RingBuf<T> {
     fn clone(&self) -> RingBuf<T> {
         self.iter().map(|t| t.clone()).collect()