{CVS ci] assign and set

[Leopold Toetsch]

I've now checked in some changes WRT these opcodes.

1) Strings - summary

  set Sx, Sy        # alias Sx to Sy
  set Sx, "str"     # set Sx to COW copy of "str"
  set Px, Sy        # CHANGED make Px refer to Sy
  assign Px, Sy     # set Px to COW copy of Sy
2) PMC summary

  set Px, Py        # alias Px to Py
  assign Px, Py     # set Px to value of Py  := Px->vtable->assign_pmc
  setref Px, Py     # make Px refer to Py    := Px->vtable->set_pmc
The latter is the opposite of deref, which now calls the get_pmc() 
vtable. C<assign> for reference PMCs is now just passed on.

leo