This PR attempts various issues around using challenges in hints, which
is blocking #1306:
1. Hints of later-phase witness columns are now removed in witgen, as
these columns don't need to be computed yet anyway and the hint might be
accessing a challenge that does not exist.
2. The query callback is now cloned for each phase of witness generation
(because otherwise it was only available in the first phase).
3. `SymbolicEvaluator` no longer panics when encountering challenges,
but returns an error. This evaluator is used to detect patterns in
identities, like `A' - A = 0`. This means that we can't detect patterns
in identities that involve challenges, but at least it doesn't panic.
4. `witgen::query_processor::Symbols` can now evaluate challenges.
5. `witgen::query_processor::Symbols` now also looks up intermediate
"polynomials" (which includes challenges). This is necessary because we
don't currently inline intermediate polynomials in hints (which we do
for identities).
I added a test that demonstrates that challenges can now be used in
hints.
This PR adds witness generation support for copy constraints: Whenever a
cell value is determined, this value is copied to all cells in its
equivalence class. This allows us to do witgen for arbitrary Plonkish
circuits (which would be detected as block machines) *as long as the
circuit is topologically sorted* (because otherwise, our row-by-row
solving strategy does not work.
Copy constraints are currently only supported in the language as
`connect` identities, as opposed to lists of cell pairs that belong to
the same equivalence class. Connecting this to the PIL input should be
part of another PR.
Fixes#844
This PR adds a new machine to the STD: `WriteOnceMemory`. This can be
used in our RISC-V machine for bootloader inputs (#1203).
Most of the issues mentioned in the issue were fixed in the meantime or
had a simple workaround (like defining `let LATCH = 1`). The only
remaining issues were in the machine detection, which I fixed here.
I also re-factor two existing tests.
With the recent changes by @pacheco, we can extract our [memory
machine](https://github.com/powdr-labs/powdr/blob/main/riscv/src/compiler.rs#L687-L841)
as a separate machine and add it to the standard library.
The result should be the same as calling the function linked above with
`with_bootloader=false`, except that the memory alignment stuff is not
inlined. For this reason, the machine is not yet used by the RISC-V
machine, but it could be after #1077 is implemented.
[This](eb320dca0c) shows the diff from
what we have in `compiler.rs`.
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