Add not_implemented_{un,bin} functions, cleanup unused "not implemented" functions

These are specifically functions for the BaseObjInst:: that need some
kind of "not implemented" function for re-use.

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

src/vm.rs

@@ -375,7 +375,7 @@ impl Vm {
                 }
                 Op::Return => {
                     let return_value = self.pop();
-                    let old_frame = self.frames.pop().unwrap();
+                    let old_frame = self.pop_frame();
                     // stack_base is always going to be <= current stack size
                     self.stack
                         .resize_with(old_frame.stack_base, || unreachable!());