Support one rule writing several targets #45

A rule has one from and one to. The text here said a value meant for several places needs several rules today; that was wrong, because validateCrosswalk rejects two rules reading the same from path, so one value could not be written to two places at all.

Decision: implement a two-way copy rule, { op: "copy", from: "items[].id", to: ["nodes[].id", "labels[].text"] }, with at least two to paths (one is a move). Encode writes the value at every to; decode reads every copy, requires them to be deeply equal and writes that value back, throwing when the copies disagree or no longer pair up element by element, since copies edited apart have no single preimage. Every to path counts towards path uniqueness and has the same wildcard count as from.

Implemented in #54.