stringify patch values -- DO NOT MERGE, BREAKING CHANGE

[phritz]

fixes #54

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[phritz]

@aboodman here's our first demonstration that we need versioning of our rpcs: if we merge this it gets pushed to production and immediately breaks existing clients in the wild. what we want to do is introduce a new field we can use if the client understands it, else use the old field (until such time as we expire old clients in the wild and then we can do away with it). we can't return both because it halves the size of the client view we can return.

for now, i'm going to introduce a version field in pullrequest and return the old encoding if 0 and the new encoding if 1. i also filed rocicorp/repc#119

[phritz]

closing in favor of #56