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)
Initialize libsodium in the gtest suite.
We left behind this initialization routine when we switched to gtest. I would rather we had moved those tests over in a separate PR instead of changing existing PRs at the last second -- we would have paid more attention to the consequences.
Run `zcash-gtest` in `make check` and fix performance tests.
* gtest tests weren't being run by make check
* performance tests were broken
* We need to automatically upload graphs of performance, [see this PR](https://github.com/Electric-Coin-Company/bbotzc/pull/15).
* Moves zerocash tests into `zcash`'s full test suite, we're removing them anyway later and it'd be nice to remove them in the PR instead of from buildbot
Taylor Hornby