Change the generation of {} and [] from 3 ops to 1, and avoid 1 mortal

Change the generation of {} and [] from 3 ops to 1, and avoid 1 mortal
on the tempstack, by augmenting pp_anonlist and pp_anonhash to accept
OPf_SPECIAL to mean "return a reference to the aggregate" on the stack
rather than the aggregate itself.

p4raw-id: //depot/perl@28771