Newtype file descriptors.

To avoid various bugs regarding the typing of file descriptors, we
newtype them into a simple wrapper type.

- Document some stuff.

kernel/scheme/debug.rs

@@ -1,7 +1,7 @@
 use core::str;
 
 use syscall::Result;
-use super::Scheme;
+use super::{Scheme, Fd};
 
 pub struct DebugScheme;
 
@@ -14,21 +14,21 @@ impl Scheme for DebugScheme {
     /// Read the file `number` into the `buffer`
     ///
     /// Returns the number of bytes read
-    fn read(&mut self, _file: usize, _buffer: &mut [u8]) -> Result<usize> {
+    fn read(&mut self, _file: Fd, _buffer: &mut [u8]) -> Result<usize> {
         Ok(0)
     }
 
     /// Write the `buffer` to the `file`
     ///
     /// Returns the number of bytes written
-    fn write(&mut self, _file: usize, buffer: &[u8]) -> Result<usize> {
+    fn write(&mut self, _file: Fd, buffer: &[u8]) -> Result<usize> {
         //TODO: Write bytes, do not convert to str
         print!("{}", unsafe { str::from_utf8_unchecked(buffer) });
         Ok(buffer.len())
     }
 
     /// Close the file `number`
-    fn close(&mut self, _file: usize) -> Result<()> {
+    fn close(&mut self, _file: Fd) -> Result<()> {
         Ok(())
     }
 }