add kill + example code

scheduler.kill seems like its not implemented comletly or doest work as
expected.

kernel/syscall/skeleton.cc

@@ -108,5 +108,8 @@ void exit(Vault &vault) {
     vault.scheduler.exit();
 }
 
+void kill(Vault &vault, size_t pid){
+    vault.scheduler.kill(&apps[pid]);
+}
 }  // namespace Skeleton
 }  // namespace Syscall