feat(kernel): add Exit and ExitThread syscalls with process cleanup

crates/kernel/src/lib.rs

@@ -125,6 +125,10 @@ impl<P: Platform> Kernel<P> {
         }
     }
 
+    pub fn process_exists(&self, pid: ProcessId) -> bool {
+        self.processes.get(pid).is_ok()
+    }
+
     fn check_cap(
         &self,
         owner: ProcessId,