Fix a lot of things and implement gc

tests/bonusheap.rs

@@ -1,8 +1,7 @@
 extern crate rustijvm;
 
 use std::fs::File;
-use std::sync::{Arc, Mutex};
-use std::io::{Cursor, Seek, SeekFrom, Read};
+use std::io::{Seek, SeekFrom, Read};
 
 fn run_bfi(file: &str) -> String {
     let file = File::open(file).expect("Missing bf file");