Fix T and F builtins to be values, not functions

* T and F were builtins that were created as functions. They have been
  changed to just be global values instead, so you don't need to suffix
  them with ! to get the creamy value inside.

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

src/vm/builtins.rs

@@ -23,15 +23,9 @@ impl MachineBuilder {
             panic!();
         });
 
-        self.register_builtin_fun("T", |machine, _| {
-            machine.stack_push(BoolObj::new(true))?;
-            Ok(BuiltinExit::Return)
-        });
-
-        self.register_builtin_fun("F", |machine, _| {
-            machine.stack_push(BoolObj::new(false))?;
-            Ok(BuiltinExit::Return)
-        });
+        // True and false values
+        self.register_global("T", BoolObj::new(true));
+        self.register_global("F", BoolObj::new(false));
 
         //
         // if