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/syscall/process.rs

@@ -1062,3 +1062,4 @@ pub fn waitpid(pid: usize, status_ptr: usize, flags: usize) -> Result<usize> {
         }
     }
 }
+