Import the non-FFI Rust sources of zkgroup

[jrose-signal]

This is the start of an effort to merge zkgroup into libsignal-client, including its Java, Swift, and TypeScript wrappers. For now we'll just concentrate on getting the Rust crate to build and pass its tests.

The first and second commits are mechanical changes that do not need review. Actually integrating zkgroup into the workspace was pretty straightforward as well:

• Bump the crate version to 0.9.0, mainly so it doesn't get confused with the original repo.
• Use the poksho in this repo and our custom 3.0.0-lizard2 branch of curve25519-dalek (instead of a 2.0.0-based one).
• Bump the sha2 dependency to match curve25519-dalek 3.0.
• Remove the reference to the crate's ffi module.