Call FFI during jet execution

[uncomputable]

Our jets should call the C code during their execution. This requires some steps:

• Merge Simplicity bindings #22 and develop the FFI further. I've been postponing this to focus on API development.
• Write a Haskell importer that automatically generates the jet initialisation files for Rust. This will bring the Rust jets back up to date. @roconnor-blockstream said he would help us with this.
• Fix any remaining incompatibilities between C and Rust. For instance, the C code focuses a lot on Elements and might not support Bitcoin alone (correct me if I'm wrong).

[sanket1729]

Addressed by #47