Accesses a global variable.
The symbol name must have been previously bound to a global variable.
This function returns that variable's current value.
Rather than calling global directly, you should usually just write name. This
will attempt to look up the global variable bound to name, unless it's currently
shadowed by a local variable.
The ? special syntax is supported. (global (? name)) will succeed, and return #n,
if name is not currently bound to a global.