Module Merkle_ledger

Any_ledger lets you use any arbitrary ledger whenever some ledger is * required. This uses dynamic dispatch and is equivalent to the notion of * consuming a value conforming to an interface in Java. * * It uses GADTs to type-erase the specific underlying first-class module * for some given signature and delegates all function calls. * * The restriction here is that one cannot conform to some signature that * exposes a `create` function because we can't magically pull a conforming * module out of thin air. However, one can always just pack any concrete * instance with the GADT constructor `witness`. * * Props to @nholland for showing me this trick. *