Serialize flaky inputs as 'json' files

[Nashtare]

When passing --save-inputs-on-error flag, we automatically write to disk the GenerationInputs that caused the proof generation to fail. The current extension is .log which is a bit confusing, especially as we serialize them into a pretty JSON string.

[Nashtare]

Note that this doesn't remove the inputs logging logic for aggregation proofs, though proving failures due to inconsistent witness (typically Wire set twice) currently yields a panic! on plonky2 side, which isn't caught by zero-bin. We should consider either removing the logging logic for aggregation proofs altogether, or refactor plonky2 proof generation to gracefully error when setting the wires.

If we lean towards the latter, I'd rather have it done soon given we'll have to do a v.1.0.0 release following the subgroup generator change anyway.