std: Stabilize last bits of io::Error

This commit stabilizes a few remaining bits of the `io::Error` type:

* The `Error::new` method is now stable. The last `detail` parameter was removed
  and the second `desc` parameter was generalized to `E: Into<Box<Error>>` to
  allow creating an I/O error from any form of error. Currently there is no form
  of downcasting, but this will be added in time.

* An implementation of `From<&str> for Box<Error>` was added to liballoc to
  allow construction of errors from raw strings.

* The `Error::raw_os_error` method was stabilized as-is.

* Trait impls for `Clone`, `Eq`, and `PartialEq` were removed from `Error` as it
  is not possible to use them with trait objects.

This is a breaking change due to the modification of the `new` method as well as
the removal of the trait implementations for the `Error` type.

[breaking-change]

src/libstd/net/udp.rs

@@ -76,7 +76,7 @@ impl UdpSocket {
         match try!(addr.to_socket_addrs()).next() {
             Some(addr) => self.0.send_to(buf, &addr),
             None => Err(Error::new(ErrorKind::InvalidInput,
-                                   "no addresses to send data to", None)),
+                                   "no addresses to send data to")),
         }
     }