Permit SET-WORD! as the argument to COPY and SET in PARSE (CC #2023)

Implementation of [CC #2023](http://curecode.org/rebol3/ticket.rsp?id=2023&cursor=23)... a seemingly uncontroversial change which allows set-words as arguments to COPY and SET in PARSE.  This is beneficial for the locals-gathering constructs like FUNCTION and CLOSURE.

    function foo [] [
        parse "Rebol" [copy name: to end]
    ]

This allows FUNCTION to find `name` and make it /LOCAL instead of leaking into the global context.

The old behavior continues to work as before.