Load KZG's trusted setup only once

[MegaRedHand]

#780 introduces the new Kzg module, but the Rust part of the NIF loads a trusted setup on every function call. We should change this so it's loaded only once at module initialization, but preferably at compile-time.

Some ideas:

• embed the file in the compiled binary (include_bytes!)
• use static variables to cache the parsed file (std::sync::Once)
• add some init function to run on module load

[Godspower-Eze]

I will unassign myself from this. I am not able to work on this atm.

I am sorry for the delay.