Fix a lot of things and implement gc

tests/advanced6.rs

@@ -1,7 +1,6 @@
 extern crate rustijvm;
 
-use std::sync::{Arc, Mutex};
-use std::io::{Cursor, Seek, SeekFrom, Read};
+use std::io::{Seek, SeekFrom, Read};
 
 fn run_calc(input: &'static str, expected: &str) {
     let rc = rustijvm::stubs::output_stub();