std: Switch stdout/stderr to buffered by default

Similarly to #12422 which made stdin buffered by default, this commit makes the
output streams also buffered by default. Now that buffered writers will flush
their contents when they are dropped, I don't believe that there's no reason why
the output shouldn't be buffered by default, which is what you want in 90% of
cases.

As with stdin, there are new stdout_raw() and stderr_raw() functions to get
unbuffered streams to stdout/stderr.

src/test/bench/shootout-fasta-redux.rs

@@ -9,7 +9,7 @@
 // except according to those terms.
 
 use std::cmp::min;
-use std::io::{stdout, BufferedWriter, IoResult};
+use std::io::{stdout, IoResult};
 use std::os;
 use std::vec::bytes::copy_memory;
 use std::vec;
@@ -183,7 +183,7 @@ fn main() {
         5
     };
 
-    let mut out = BufferedWriter::new(stdout());
+    let mut out = stdout();
 
     out.write_line(">ONE Homo sapiens alu").unwrap();
     {

src/test/bench/shootout-fasta.rs

@@ -117,6 +117,6 @@ fn main() {
         let mut file = BufferedWriter::new(File::create(&Path::new("./shootout-fasta.data")));
         run(&mut file);
     } else {
-        run(&mut BufferedWriter::new(io::stdout()));
+        run(&mut io::stdout());
     }
 }

src/test/bench/shootout-mandelbrot.rs

@@ -9,7 +9,6 @@
 // except according to those terms.
 
 use std::io;
-use std::io::BufferedWriter;
 
 struct DummyWriter;
 impl Writer for DummyWriter {
@@ -27,7 +26,7 @@ fn main() {
         (1000, ~DummyWriter as ~Writer)
     } else {
         (from_str(args[1]).unwrap(),
-         ~BufferedWriter::new(std::io::stdout()) as ~Writer)
+         ~std::io::stdout() as ~Writer)
     };
     let h = w;
     let mut byte_acc = 0u8;