Some integration with Type Checking.

Now the UI highlights commands and numbers as you move the mouse, numbers
are blue, commands that type-check are green, commands that fail to
type-check are orange and will not be interpreted, and if there is no
stack effect information available for a command it is grey but you can
still attempt to execute it.

You can still evaluate whole expressions by selceting them and
right-inter-clicking before you release the left button, or by putting
the cursor on a line and typing ctrl-enter, which will run the whole
line.  These expressions are NOT (yet) type-checked.

joy/gui/main.py

@@ -19,10 +19,10 @@ from joy.utils.stack import stack_to_string
 
 tb = TEXT_BINDINGS.copy()
 tb.update({
-  '<F4>': lambda tv: tv.cut,
   '<F3>': lambda tv: tv.copy_selection_to_stack,
+  '<F4>': lambda tv: tv.cut,
   #  '<F-->': lambda tv: tv.pastecut,
-  '<F6>': lambda tv: tv.copyto,
+  #  '<F6>': lambda tv: tv.copyto,
   })
 defaults = dict(text_bindings=tb, width=80, height=25)