SDK for guest programs

[matthiasgoergens]

We want to make it easy and pleasant for people to write guest programs. We mainly support using Rust, but as a proof of concept we should also support running C programs. (Mostly so we don't get caught up by Rust-only idiosyncrasies.)

• Rust standard library for guest programs (lean on SP1 and Risc0) #684
• Build system settings and scripts ('toolchain')
  • Consider a cargo plugin like Risc0 and SP1 (but hopefully we can get away without)
  • Consider whether we need a Rust fork (like Risc0 and SP1, but try to avoid it)
• IO:
  • Design of API from the guest's point of view (ceno_rt) #681
  • VM support for IO primitives (ceno_emul) #682
  • Circuit support for IO primitives (ceno_zkvm) #683
• Precompiles (like hash acceleration)
  • Design of API from the guest's point of view
  • VM support
  • Circuit Support