Port to latest Iris and make use of the `PureExec` infrastructure.

opam

theories/lang/lang.v

theories/lang/lib/arc.v

theories/lang/lib/memcpy.v

theories/lang/lib/new_delete.v

theories/lang/lib/spawn.v

theories/lang/lib/swap.v

theories/lang/lib/tests.v

theories/lang/lifting.v

theories/lang/proofmode.v

theories/lang/tactics.v

theories/typing/base.v

theories/typing/bool.v

theories/typing/cont.v

theories/typing/function.v

theories/typing/int.v

theories/typing/lib/arc.v

theories/typing/lib/fake_shared_box.v

theories/typing/lib/panic.v

theories/typing/lib/rc/rc.v