Move MethodInst stack mangling to MethodInst::call

When a MethodInst is called as a function in the VM, it needs to push
its `self_binding` member to the stack. Previously, we were downcasting
(if possible) to MethodInst in the VM, but really, we are calling
`MethodInst::call` anyway, so it makes more sense to do
MethodInst-specific stuff in the MethodInst-specific function.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

src/obj.rs

@@ -1079,10 +1079,16 @@ impl Display for MethodInst {
 
 impl Obj for MethodInst {
     fn arity(&self) -> Option<Argc> {
-        self.function.borrow().arity()
+        // Subtract one from the arity - this is because the VM uses arity() to check against the
+        // number of arguments passed.
+        self.function.borrow().arity().map(|arity| arity - 1)
     }
 
-    fn call(&self, vm: &mut Vm, argc: Argc) {
+    fn call(&self, vm: &mut Vm, mut argc: Argc) {
+        let self_pos = vm.stack().len() - (argc as usize);
+        vm.stack_mut().insert(self_pos, self.self_binding.clone());
+        argc += 1;
+
         self.function.borrow().call(vm, argc)
     }