Extract useful parts of the Sapling security analysis into the Orchard book

[ebfull]

There may be some useful pieces of the old Sapling security analysis that would be useful to extract and place into the Orchard book. I don't think this is urgent; we never had that document analyzed (it was never finished) and it was not an exhaustive account of the security properties we needed, just an attempt to begin capturing those details into a formal framework. More work on strengthening these properties and continuing to develop these proofs could help us analyze Sapling and Orchard's security better.