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/event.rs

@@ -3,6 +3,7 @@ use collections::BTreeMap;
 use spin::{Once, RwLock, RwLockReadGuard, RwLockWriteGuard};
 
 use context;
+use scheme::SchemeId;
 use sync::WaitQueue;
 use syscall::data::Event;
 
@@ -10,7 +11,7 @@ type EventList = Weak<WaitQueue<Event>>;
 
 #[derive(PartialEq, Eq, PartialOrd, Ord)]
 pub struct RegKey {
-    scheme_id: usize,
+    scheme_id: SchemeId,
     event_id: usize,
 }
 
@@ -39,7 +40,7 @@ pub fn registry_mut() -> RwLockWriteGuard<'static, Registry> {
     REGISTRY.call_once(init_registry).write()
 }
 
-pub fn register(fd: usize, scheme_id: usize, event_id: usize) -> bool {
+pub fn register(fd: usize, scheme_id: SchemeId, event_id: usize) -> bool {
     let (context_id, events) = {
         let contexts = context::contexts();
         let context_lock = contexts.current().expect("event::register: No context");
@@ -66,7 +67,7 @@ pub fn register(fd: usize, scheme_id: usize, event_id: usize) -> bool {
     }
 }
 
-pub fn unregister(fd: usize, scheme_id: usize, event_id: usize) {
+pub fn unregister(fd: usize, scheme_id: SchemeId, event_id: usize) {
     let mut registry = registry_mut();
 
     let mut remove = false;
@@ -91,7 +92,7 @@ pub fn unregister(fd: usize, scheme_id: usize, event_id: usize) {
     }
 }
 
-pub fn trigger(scheme_id: usize, event_id: usize, flags: usize, data: usize) {
+pub fn trigger(scheme_id: SchemeId, event_id: usize, flags: usize, data: usize) {
     let registry = registry();
     let key = RegKey {
         scheme_id: scheme_id,

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