Auto merge of #908 - ebfull:trafford, r=ebfull

libzcash and new zkSNARK circuit implementation

This PR completes [`libzcash`](https://github.com/zcash/zcash/tree/zc.v0.11.2.latest/src/zcash), the implementation of the [Zcash protocol specification](https://github.com/zcash/zips/blob/9bb4410e45394c8b7e43e35622f54dd4041a4ddd/protocol/protocol.pdf) and replacement of [`libzerocash`](https://github.com/Zerocash/libzerocash), our old Zerocash protocol implementation. The new spec comes with some improvements to security and terminology, with minimal differences from the original academic design.

This implementation includes:

* A rewrite of the zkSNARK circuit for `JoinSplit` operations. This rewrite is cleaner, broken up into separate gadgets, easier to audit and review, and fixes some security bugs. (Closes #822, Closes #809, Closes #500, Closes #854)
* A minimal API for interacting with `JoinSplit`s and surrounding primitives. This PR removes almost twice as much code as it introduces. (Closes #877, Closes #315, Closes #824, Closes #798, Closes #707, Closes #512, Closes #247, Closes #128, Closes #514)