auto merge of #8555 : chris-morgan/rust/time-clone, r=huonw

I need `Clone` for `Tm` for my latest work on [rust-http](https://github.com/chris-morgan/rust-http) (static typing for headers, and headers like `Date` are a time), so here it is.

@huonw recommended deriving DeepClone while I was at it.

I also had to implement `DeepClone` for `~str` to get a derived implementation of `DeepClone` for `Tm`; I did `@str` while I was at it, for consistency.

src/libstd/str.rs

@@ -20,7 +20,7 @@ use at_vec;
 use cast;
 use char;
 use char::Char;
-use clone::Clone;
+use clone::{Clone, DeepClone};
 use container::{Container, Mutable};
 use iter::Times;
 use iterator::{Iterator, FromIterator, Extendable};
@@ -2104,6 +2104,13 @@ impl Clone for ~str {
     }
 }
 
+impl DeepClone for ~str {
+    #[inline]
+    fn deep_clone(&self) -> ~str {
+        self.to_owned()
+    }
+}
+
 impl Clone for @str {
     #[inline]
     fn clone(&self) -> @str {
@@ -2111,6 +2118,13 @@ impl Clone for @str {
     }
 }
 
+impl DeepClone for @str {
+    #[inline]
+    fn deep_clone(&self) -> @str {
+        *self
+    }
+}
+
 impl FromIterator<char> for ~str {
     #[inline]
     fn from_iterator<T: Iterator<char>>(iterator: &mut T) -> ~str {