Try to reduce write(3) calls for OSC 133 prompt markers

Something like

	write!(f, "foo{}bar", ...)

seems to call f.write_str() thrice.

Splitting a single OSC 133 command into three calls to write(3) might result in
odd situations if one of them fails. Let's try to do it in one in most cases.

Add a new buffered output type that can be used with write!(). This is
somewhat redundant given that we have scoped_buffer().  While at it, remove
the confused error handling.  This doesn't fail unless we are OOM (and this
new type makes that more obvious).

src/output.rs

@@ -456,6 +456,35 @@ pub fn stdoutput() -> &'static RefCell<Outputter> {
     }
 }
 
+pub struct BufferedOuputter<'a>(&'a mut Outputter);
+
+impl<'a> BufferedOuputter<'a> {
+    pub fn new(outputter: &'a mut Outputter) -> Self {
+        outputter.begin_buffering();
+        Self(outputter)
+    }
+}
+
+impl<'a> Drop for BufferedOuputter<'a> {
+    fn drop(&mut self) {
+        self.0.end_buffering();
+    }
+}
+
+impl<'a> Write for BufferedOuputter<'a> {
+    fn write(&mut self, buf: &[u8]) -> Result<usize> {
+        self.0
+            .write(buf)
+            .expect("Writing to in-memory buffer should never fail");
+        Ok(buf.len())
+    }
+
+    fn flush(&mut self) -> Result<()> {
+        self.0.flush().unwrap();
+        Ok(())
+    }
+}
+
 /// Given a list of RgbColor, pick the "best" one, as determined by the color support. Returns
 /// RgbColor::NONE if empty.
 pub fn best_color(candidates: &[RgbColor], support: ColorSupport) -> RgbColor {

src/reader.rs

@@ -704,11 +704,12 @@ fn read_i(parser: &Parser) -> i32 {
         parser.libdata_mut().exit_current_script = false;
 
         // OSC 133 "Command finished"
-        let _ = write!(
-            Outputter::stdoutput().borrow_mut(),
+        write!(
+            BufferedOuputter::new(&mut Outputter::stdoutput().borrow_mut()),
             "\x1b]133;D;{}\x07",
             parser.get_last_status()
-        );
+        )
+        .unwrap();
         event::fire_generic(parser, L!("fish_postexec").to_owned(), vec![command]);
         // Allow any pending history items to be returned in the history array.
         data.history.resolve_pending();