core::rt: Make I/O constructors return Option instead of Result

For consistency, for all I/O calls, inspecting the error can be done with
the io_error condition.

src/libcore/rt/io/file.rs

@@ -42,7 +42,7 @@ impl FileStream {
     pub fn open<P: PathLike>(_path: &P,
                              _mode: FileMode,
                              _access: FileAccess
-                            ) -> Result<FileStream, IoError> {
+                            ) -> Option<FileStream> {
         fail!()
     }
 }

src/libcore/rt/io/net/tcp.rs

@@ -16,7 +16,7 @@ use super::ip::IpAddr;
 pub struct TcpStream;
 
 impl TcpStream {
-    pub fn connect(_addr: IpAddr) -> Result<TcpStream, IoError> {
+    pub fn connect(_addr: IpAddr) -> Option<TcpStream> {
         fail!()
     }
 }
@@ -40,7 +40,7 @@ impl Close for TcpStream {
 pub struct TcpListener;
 
 impl TcpListener {
-    pub fn bind(_addr: IpAddr) -> Result<TcpListener, IoError> {
+    pub fn bind(_addr: IpAddr) -> Option<TcpListener> {
         fail!()
     }
 }

src/libcore/rt/io/net/udp.rs

@@ -16,7 +16,7 @@ use super::ip::IpAddr;
 pub struct UdpStream;
 
 impl UdpStream {
-    pub fn connect(_addr: IpAddr) -> Result<UdpStream, IoError> {
+    pub fn connect(_addr: IpAddr) -> Option<UdpStream> {
         fail!()
     }
 }
@@ -40,7 +40,7 @@ impl Close for UdpStream {
 pub struct UdpListener;
 
 impl UdpListener {
-    pub fn bind(_addr: IpAddr) -> Result<UdpListener, IoError> {
+    pub fn bind(_addr: IpAddr) -> Option<UdpListener> {
         fail!()
     }
 }

src/libcore/rt/io/net/unix.rs

@@ -16,7 +16,7 @@ use super::super::support::PathLike;
 pub struct UnixStream;
 
 impl UnixStream {
-    pub fn connect<P: PathLike>(_path: &P) -> Result<UnixStream, IoError> {
+    pub fn connect<P: PathLike>(_path: &P) -> Option<UnixStream> {
         fail!()
     }
 }
@@ -40,7 +40,7 @@ impl Close for UnixStream {
 pub struct UnixListener;
 
 impl UnixListener {
-    pub fn bind<P: PathLike>(_path: &P) -> Result<UnixListener, IoError> {
+    pub fn bind<P: PathLike>(_path: &P) -> Option<UnixListener> {
         fail!()
     }
 }