Remove dependencies by switching to simplified API with a callback interface

[conradoplg]

zcash_script links in zcashd source code, and since the API takes a CTransaction that requires include a bunch of other stuff like orchard, etc.

This simplifies the API, by not passing the entire transaction but only the data required to verify. This makes this crate have ZERO Rust dependencies 🎉

This requires using a callback to compute the sighash, since to compute it you need some information that is extracted with zcash_script.cpp.

This approach does not decrease performance because it still allows the use of precomputation in the sighash: simply precompute and store in the ctx pointer, which will be forwarded to the sighash callback and can be used in the sighash computation by the caller.