Emit chain id in the app circuit public inputs and validate in the kernel circuit

[spalladino]

To prevent the same proof from being submitted to more than one chain, we should include the chain id in the app circuit public inputs. So, when the user creates a ZKP for their tx, they just include the intended chain id as part of the public inputs. This value should then be validated by the kernel circuit.

TBD: Is it enough that the kernel circuit validates it? And we have a different kernel circuit for each chain, with the chain id hardcoded?

Raised by @LeilaWang in #656 (comment).

[iAmMichaelConnor]

I believe this is complete.