Implementation of zk-SNARK proofs.

This is the fork of the original [`bellperson`][bellperson] crate from
Filecoin, which itself is a fork of the [`bellman`][bellman] crate.

## Delta from upstream

These are the difference between this crate and the upstream
[`bellperson`][bellperson] crate:

* Made some arguments to the methods in the `boolean` module more generic.

[bellperson]: https://github.com/filecoin-project/bellman
[bellman]: https://github.com/zkcrypto/bellman