std,extra: Make some types public and other private.

These are either returned from public functions, and really should
appear in the documentation, but don't since they're private, or are
implementation details that are currently public.

src/libstd/io/mod.rs

@@ -1268,7 +1268,7 @@ pub trait Acceptor<T> {
 /// The Some contains another Option representing whether the connection attempt was succesful.
 /// A successful connection will be wrapped in Some.
 /// A failed connection is represented as a None and raises a condition.
-struct IncomingConnections<'a, A> {
+pub struct IncomingConnections<'a, A> {
     priv inc: &'a mut A,
 }