Feature request: init librustzcash without sprout-groth16

[zcash]

Feature request: init librustzcash without sprout-groth16

[str4d]

The core Rust changes for this were made in #65, so all that is necessary is to enable librustzcash_init_zksnark_params() to be called without the Sprout parameters. It should be doable by interpreting a null pointer for the Sprout parameter path as "we don't want this one loaded".

At the same time, we should internalise the hashes instead of passing them in, because they aren't ever going to change (passing them through was only beneficial during development prior to the MPC). This would simplify the librustzcash_init_zksnark_params() API to just be the paths.

[str4d]

Opened #106 for internalising the hashes.

[leto]

FYI, HUSH maintains a trivial patch on top of the stock librustzcash to prevent the need for sprout-groth16, so our itch is scratched:

https://github.com/MyHush/hush3/blob/master/depends/patches/librustzcash/no-groth16.patch