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/scheme/sys/context.rs

@@ -83,8 +83,8 @@ pub fn resource() -> Result<Vec<u8>> {
             let name = str::from_utf8(&name_bytes).unwrap_or("");
 
             string.push_str(&format!("{:<6}{:<6}{:<6}{:<6}{:<6}{:<6}{:<8}{}\n",
-                               context.id,
-                               context.ppid,
+                               context.id.into(),
+                               context.ppid.into(),
                                context.euid,
                                context.egid,
                                stat_string,

kernel/scheme/user.rs

@@ -53,7 +53,7 @@ impl UserInner {
 
         self.call_inner(Packet {
             id: self.next_id.fetch_add(1, Ordering::SeqCst),
-            pid: pid,
+            pid: pid.into(),
             uid: uid,
             gid: gid,
             a: a,
@@ -297,7 +297,7 @@ impl Scheme for UserScheme {
 
         inner.call_inner(Packet {
             id: id,
-            pid: pid,
+            pid: pid.into(),
             uid: uid,
             gid: gid,
             a: SYS_FMAP,