Ensure that binary series constructed from blocks are distinct

(Addresses [Cure Code #1967](http://curecode.org/rebol3/ticket.rsp?id=1967).)

When passed a block argument, `make_binary` populates and returns a pointer to a temporary buffer in the task context.  As a result, binary series constructed from block specs are subject to unexpected mutation:

    >> x: to binary! [1]
    == #{01}
    >> y: to binary! [2]
    == #{02}
    >> x
    == #{02}
    >> same? to binary! [1] to binary! [2]
    == true

Correct this behaviour by returning a copy of the constructed series.