Return a buffered stdin by default.

One of the most common ways to use the stdin stream is to read it line by line
for a small program. In order to facilitate this common usage pattern, this
commit changes the stdin() function to return a BufferedReader by default. A new
`stdin_raw()` method was added to get access to the raw unbuffered stream.

I have not changed the stdout or stderr methods because they are currently
unable to flush in their destructor, but #12403 should have just fixed that.

src/libstd/io/mod.rs

@@ -26,11 +26,9 @@ Some examples of obvious things you might want to do
 * Read lines from stdin
 
     ```rust
-    use std::io::BufferedReader;
-    use std::io::stdin;
+    use std::io;
 
-    let mut stdin = BufferedReader::new(stdin());
-    for line in stdin.lines() {
+    for line in io::stdin().lines() {
         print!("{}", line);
     }
     ```
@@ -1097,10 +1095,9 @@ pub trait Buffer: Reader {
     /// # Example
     ///
     /// ```rust
-    /// use std::io::{BufferedReader, stdin};
-    ///
-    /// let mut reader = BufferedReader::new(stdin());
+    /// use std::io;
     ///
+    /// let mut reader = io::stdin();
     /// let input = reader.read_line().ok().unwrap_or(~"nothing");
     /// ```
     ///