auto merge of #5630 : erickt/rust/serial, r=erickt

@nikomatsakis and I were talking about how the serializers were a bit too complicated. None of the users of With the `emit_option` and `read_option` functions, the serializers are now moving more high level. This patch series continues that trend. I've removed support for emitting specific string and vec types, and added support for emitting mapping types.

src/libsyntax/ast.rs

@@ -71,7 +71,7 @@ impl<S:Encoder> Encodable<S> for ident {
             Some(intr) => intr
         };
 
-        s.emit_owned_str(*(*intr).get(*self));
+        s.emit_str(*(*intr).get(*self));
     }
 }
 
@@ -84,7 +84,7 @@ impl<D:Decoder> Decodable<D> for ident {
             Some(intr) => intr
         };
 
-        (*intr).intern(@d.read_owned_str())
+        (*intr).intern(@d.read_str())
     }
 }