Converting scheme ids into a new type SchemeId

Keeping scheme ids (and pids, and file handles, ...) as usize is a
footgun. Let's remove it.

kernel/context/file.rs

@@ -1,11 +1,13 @@
 //! File struct
 
+use scheme::SchemeId;
+
 /// A file
 //TODO: Close on exec
 #[derive(Copy, Clone, Debug)]
 pub struct File {
     /// The scheme that this file refers to
-    pub scheme: usize,
+    pub scheme: SchemeId,
     /// The number the scheme uses to refer to this file
     pub number: usize,
     /// If events are on, this is the event ID