Rename AlwaysStable into Persistent as suggested by Derek.

algebra/upred.v

algebra/upred_big_op.v

barrier/proof.v

heap_lang/heap.v

program_logic/auth.v

program_logic/ghost_ownership.v

program_logic/invariants.v

program_logic/ownership.v

program_logic/pviewshifts.v

program_logic/saved_prop.v

program_logic/sts.v

program_logic/weakestpre.v