Fix a lot of things and implement gc

src/machine.rs

@@ -11,7 +11,7 @@ use stack::Stack;
 use Result;
 
 #[cfg(feature = "bonus:heap")]
-use heap::Heap;
+use heap::Heaps;
 #[cfg(feature = "bonus:network")]
 use netstack::NetStack;
 use std::convert::TryInto;
@@ -28,7 +28,7 @@ pub struct Machine {
     #[cfg(feature = "bonus:network")]
     pub net: NetStack,
     #[cfg(feature = "bonus:heap")]
-    pub heap: Heap,
+    pub heap: Heaps,
 
     pub stream_in: Box<Read + Send + Sync>,
     pub stream_out: Arc<Mutex<Write + Send + Sync>>,
@@ -49,7 +49,7 @@ impl Machine {
             net: NetStack::new(),
 
             #[cfg(feature = "bonus:heap")]
-            heap: Heap::new(),
+            heap: Heaps::new(),
         }
     }