Add Read::initializer.

This is an API that allows types to indicate that they can be passed
buffers of uninitialized memory which can improve performance.

src/libstd/process.rs

@@ -60,7 +60,7 @@ use io::prelude::*;
 use ffi::OsStr;
 use fmt;
 use fs;
-use io;
+use io::{self, Initializer};
 use path::Path;
 use str;
 use sys::pipe::{read2, AnonPipe};
@@ -208,8 +208,9 @@ impl Read for ChildStdout {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
         self.inner.read(buf)
     }
-    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
-        self.inner.read_to_end(buf)
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        Initializer::nop()
     }
 }
 
@@ -250,8 +251,9 @@ impl Read for ChildStderr {
     fn read(&mut self, buf: &mut [u8]) -> io::Result<usize> {
         self.inner.read(buf)
     }
-    fn read_to_end(&mut self, buf: &mut Vec<u8>) -> io::Result<usize> {
-        self.inner.read_to_end(buf)
+    #[inline]
+    unsafe fn initializer(&self) -> Initializer {
+        Initializer::nop()
     }
 }