Add a new predefined constant.
This function is often used to add builtin functions.
All programs compiled after the add_constant() function has been
called can access value by the name name .
If there is a constant called name already, it will be replaced by
by the new definition. This will not affect already compiled programs.
Calling add_constant() without a value will remove that name from
the list of constants. As with replacing, this will not affect already
compiled programs.