finish making step-index a typeclass

theories/base_logic/algebra.v

theories/base_logic/bi.v

theories/base_logic/derived.v

theories/base_logic/lib/cancelable_invariants.v

theories/base_logic/lib/fancy_updates.v

theories/base_logic/lib/gen_heap.v

theories/base_logic/lib/invariants.v

theories/base_logic/lib/iprop.v

theories/base_logic/lib/na_invariants.v

theories/base_logic/lib/own.v

theories/base_logic/lib/proph_map.v

theories/base_logic/lib/satisfiable.v

theories/base_logic/lib/saved_prop.v

theories/base_logic/lib/wsat.v

theories/base_logic/proofmode.v

theories/base_logic/upred.v

theories/bi/lib/fixpoint.v

theories/bi/lib/fractional.v

theories/proofmode/class_instances.v

theories/proofmode/class_instances_embedding.v