Add internal error handling to VM, plus function arity

* VM is able to handle basic runtime errors although there is no way to
  catch this in executing code currently
* If a function is called with the wrong number of arguments (arity) it
  will invoke a runtime error.

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

src/compile/mod.rs

@@ -42,7 +42,7 @@ impl Compile {
                 sym
             })
             .collect();
-        Ok((self.const_data.const_pool, UserFun::new_obj(main, globals)))
+        Ok((self.const_data.const_pool, UserFun::new_obj(main, globals, 0)))
     }
 
     /// Gets the constant data that is interned in this compile session.

src/compile/thunk.rs

@@ -407,7 +407,7 @@ impl Visit for CompileBody<'_> {
             })
             .collect();
 
-        let (hdl, _fun) = self.compile.push_const(UserFun::new_obj(code, locals));
+        let (hdl, _fun) = self.compile.push_const(UserFun::new_obj(code, locals, expr.params.len()));
 
         // TODO(compile) : determine return value at the end of the body (preferably at parse-time)