Array.v: prove updN_firstn_comm (updN and firstn commute, when in-bounds)
Array.v: prove updN_firstn_comm (updN and firstn commute, when in-bounds)

mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl,mit-pdos/fscq-impl