core: Implement unwrap()/unwrap_err() on Result

Now that std::fmt is in libcore, it's possible to implement this as an inherit
method rather than through extension traits.

This commit also tweaks the failure interface of libcore to libstd to what it
should be, one method taking &fmt::Arguments

src/libstd/lib.rs

@@ -153,6 +153,7 @@ pub use core::mem;
 pub use core::ptr;
 pub use core::raw;
 pub use core::tuple;
+pub use core::result;
 
 // Run tests with libgreen instead of libnative.
 //
@@ -218,7 +219,6 @@ pub mod hash;
 
 /* Common data structures */
 
-pub mod result;
 pub mod option;
 
 /* Tasks and communication */