cursor write revert to memcpy

src/libio/cursor.rs

@@ -62,12 +62,8 @@ impl Write for Cursor<&mut [u8]> {
     fn write(&mut self, buf: &[u8]) -> Result<usize, IoError> {
         let data = &mut self.inner[self.pos..];
         let len = buf.len().min(data.len());
-        for i in 0..len {
-            unsafe {
-                asm!("", options(preserves_flags, nostack, readonly));
-            }
-            data[i] = buf[i];
-        }
+        // as long as 'copy_work_buffer' is used in drtioaux, memcpy here is allowed
+        data[..len].copy_from_slice(&buf[..len]);
         self.pos += len;
         Ok(len)
     }