Dir.v: first take at an invariant + proof for [dfold]
Dir.v: first take at an invariant + proof for [dfold]

some [admit]s left for lengths / overflow conditions..

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