Extract Sapling protocol into sapling-crypto crate

[str4d]

Back when the Sapling protocol was first developed, it lived in two places:

• A sapling-crypto crate for the circuit logic.
• A librustzcash crate that exposed the circuit logic to zcashd via FFI methods that contained a bunch of logic.

We refactored these to produce the zcash_* crates, and eventually got rid of both of the above crates (sapling-crypto entirely, librustzcash moved into zcashd as a pure FFI layer).

However, with the introduction of the orchard crate (which is kept in a separate repository for licensing reasons), we now run into a dependency problem: the orchard crate depends on zcash_note_encryption which is in this workspace, and zcash_primitives depends on orchard. This makes updates very tricky.

Instead, we want to fix this by moving the components/* libraries (everything that exists "below" the shielded protocols) out of this workspace, and also move the Sapling protocol into a separate crate to emulate the orchard crate positioning in the dependency graph. We can use this as an opportunity to resurrect the sapling-crypto crate!

This issue is not for moving sapling-crypto back into its old GitHub repository (the dependency issue only requires moving the component crates). We may still choose to do that, but it would be after this issue.

[str4d]

This issue is not for moving sapling-crypto back into its old GitHub repository (the dependency issue only requires moving the component crates). We may still choose to do that, but it would be after this issue.

We have decided to do this, so we'll do so at the same time as this issue.