Functions are implemented and VM should be able to handle function calls

* VM Signals are used by running functions to dictate whether a function
  should return, or if it should call another function. These signals
  can be injected at any time allowing for user functions to inject
  themselves at runtime.
* obj::Method is gone since it's not being used yet.
* Obj impls must implement as_any(&self) -> &dyn Any now. This allows
  for UserFun and NativeFun to be explicitly cast (among other things,
  in the future).

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

src/vm/frame.rs

@@ -4,8 +4,7 @@ use shredder::{GcSafe, Scan, Scanner};
 /// A stack call frame.
 #[derive(Debug, Clone)]
 pub enum FrameKind {
-    Root,
-    Native,
+    Native(NativeFunRef, Vec<ObjRef>),
     User {
         last_pc: usize,
         stack_base: usize,