message: Return the number of bytes written in sys_write. Fixes infinite loop in hellof.
diff: use console;
  use core::fmt::Write;
  -use core::{slice, str};
  +use core::{isize, slice, str};
  use errno::*;
  @@ -47,13 +47,14 @@ pub extern "C" fn sys_read(fd: i32, buf: *mut u8, len: usize) -> isize {
  #[no_mangle]
  pub extern "C" fn sys_write(fd: i32, buf: *const u8, len: usize) -> isize {
  info!("sys_write is halfplemented");
  + assert!(len <= isize::MAX as usize);
  unsafe {
  let slice = slice::from_raw_parts(buf, len);
  console::CONSOLE.lock().write_str(str::from_utf8_unchecked(slice)).unwrap();
  }
  - 0
  + len as isize
  }
  #[no_mangle]
  