Rec.v: avoid constructing [let] terms in the first place
Rec.v: avoid constructing [let] terms in the first place

Instead of trying to replace [let] terms with [fst] and [snd] after the
fact, change [recget] and [recset] to avoid constructing such terms in
the first place.

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