refactor memory management code

We now support immortal objects, which the GC will scan for references
but not consider for collection.  On x86_64, we allocate JIT code memory
via mmap, which lets us map memory into the bottom 2GB of the address
space, ensuring that 32-bit relative jumps and calls work.

src/interpret.cpp

@@ -3015,8 +3015,8 @@ class MyProcessor: public Processor {
     }
   }
 
-  virtual void dispose(vm::Thread*) {
-    // ignore
+  virtual void dispose(vm::Thread* t) {
+    s->free(t, sizeof(Thread));
   }
 
   virtual void dispose() {