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/io/cursor.rs

@@ -75,8 +75,7 @@ macro_rules! seek {
 
             if pos < 0 {
                 Err(Error::new(ErrorKind::InvalidInput,
-                               "invalid seek to a negative position",
-                               None))
+                               "invalid seek to a negative position"))
             } else {
                 self.pos = pos as u64;
                 Ok(self.pos)