Converting pids into a new type ContextId

Keeping pid (and file descriptor, and scheme id, ...) as usize is a
footgun. Let's remove it.

kernel/context/event.rs

@@ -17,7 +17,7 @@ pub struct RegKey {
 
 #[derive(PartialEq, Eq, PartialOrd, Ord)]
 pub struct ProcessKey {
-    context_id: usize,
+    context_id: context::context::ContextId,
     fd: usize,
 }