Introduce AsyncProver

[plafer]

Based on #271.

Introduced an asynchronous version of Prover (under a new "async" feature). A few notes:

• Uses the new return-position impl Trait in traits of Rust 1.75. I chose not to use the async syntactic sugar, as it doesn't allow us to specify + Send + Sync on the return Futures (as explained in the linked blog post).
  • This is a pragmatic solution, since the compiler team currently recommends using the trait-variant crate, but the trait_variant macro didn't like our E: FieldElement<BaseField = Self::BaseField> bound in generate_proof() (with a difficult to decipher error message).
  • Also, I'm not sure that I like generating another trait just to get + Send bounds. So I decided to only support the + Send + Sync variant, so that it would work with tokio's spawn().
• We should fix the tracing spans in a few spots if we decide to go down this route
• After Generalize auxiliary trace building #271 is merged, I would split Prover::generate_proof() into many sub- functions, and call the appropriate ones in AsyncProver::generate_proof() so as to remove the amount of copy/paste code
• Personal TODO: make sure the CI will properly build/check the new "async" feature

I'm happy to revisit any of these assumptions 🙂.

[plafer]

Closing as we will go in a different direction