std: Add I/O timeouts to networking objects

These timeouts all follow the same pattern as established by the timeouts on
acceptors. There are three methods: set_timeout, set_read_timeout, and
set_write_timeout. Each of these sets a point in the future after which
operations will time out.

Timeouts with cloned objects are a little trickier. Each object is viewed as
having its own timeout, unaffected by other objects' timeouts. Additionally,
timeouts do not propagate when a stream is cloned or when a cloned stream has
its timeouts modified.

This commit is just the public interface which will be exposed for timeouts, the
implementation will come in later commits.

src/libstd/io/mod.rs

@@ -434,6 +434,17 @@ pub enum IoErrorKind {
     InvalidInput,
     /// The I/O operation's timeout expired, causing it to be canceled.
     TimedOut,
+    /// This write operation failed to write all of its data.
+    ///
+    /// Normally the write() method on a Writer guarantees that all of its data
+    /// has been written, but some operations may be terminated after only
+    /// partially writing some data. An example of this is a timed out write
+    /// which successfully wrote a known number of bytes, but bailed out after
+    /// doing so.
+    ///
+    /// The payload contained as part of this variant is the number of bytes
+    /// which are known to have been successfully written.
+    ShortWrite(uint),
 }
 
 /// A trait for objects which are byte-oriented streams. Readers are defined by
@@ -1429,7 +1440,8 @@ pub fn standard_error(kind: IoErrorKind) -> IoError {
         PathDoesntExist => "no such file",
         MismatchedFileTypeForOperation => "mismatched file type",
         ResourceUnavailable => "resource unavailable",
-        TimedOut => "operation timed out"
+        TimedOut => "operation timed out",
+        ShortWrite(..) => "short write",
     };
     IoError {
         kind: kind,