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>

examples/branch.sy

@@ -1,7 +1,10 @@
-0 [ "fail - 0 is not truthy" println! ] [ "OK" println! ] if!
-1 [ "OK" println! ] [ "fail - 1 is truthy" println! ] if!
-1.1 [ "OK" println! ] [ "fail - 1.1 is truthy" println! ] if!
-0.1 [ "OK" println! ] [ "fail - 0.1 is truthy" println! ] if!
--0.1 [ "OK" println! ] [ "fail - -0.1 is truthy" println! ] if!
--0.0 [ "fail - -0.0 is not truthy" println! ] [ "OK" println! ] if!
-0.0 [ "fail - 0.0 is not truthy" println! ] [ "OK" println! ] if!
+0 [ "fail - 0 is not truthy" ] [ "OK" ] if! println!
+1 [ "OK" ] [ "fail - 1 is truthy" ] if! println!
+1.1 [ "OK" ] [ "fail - 1.1 is truthy" ] if! println!
+0.1 [ "OK" ] [ "fail - 0.1 is truthy" ] if! println!
+-0.1 [ "OK" ] [ "fail - -0.1 is truthy" ] if! println!
+-0.0 [ "fail - -0.0 is not truthy" ] [ "OK" ] if! println!
+0.0 [ "fail - 0.0 is not truthy" ] [ "OK" ] if! println!
+
+T [ "OK" ] [ "fail - T should push True to the stack" ] if! println!
+F [ "fail - F should push False to the stack" ] [ "OK" ] if! println!

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