Sort out how to include zcash code

[conradoplg]

We need to include zcash source code in order to compile and wrap it. But each strategy has downsides:

• submodules prevents us from packaging the crate because cargo package ignores subfolders with Cargo.toml, and zcash has one in the root
• subtree works but requires us using merge commits (which we try to avoid in our repos), since subtree relies on them to work

Come up with a better way to solve this and implement it.

Ideas:

• Make submodules work with sparse-checkout so that we only checkout the src subfolder (which is what we need, and it does not contain a Cargo.toml
• Find a way to automatically remove the Cargo.toml before packaging the crate
• Something else?