Prover-function based fast witgen

[chriseth]

We would like to have prover-function based fast witgen. This issue is not about auto-generating prover functions [-> #2066], it assumes correct prover functions are provided by the user. We can roughly break it down into the following steps:

1. allow access (read and write) to all rows in a block of a block machine
2. potentially modify the whole pattern of how new values are provided in the prover function evaluation process
3. implement the required built-in functions
4. implement missing pieces for JIT if needed
5. find a mechanism to directly return the result of a query to a sub-machine, so that filling the actual cells in the submachines can be done in a separate thread

Related explorative PR with some numbers: #2028

[chriseth]

We implemented this without going through pil. We might revisit this at a later point in time, first auto-generating prover functions and then compiling pil to rust.