Cleanup, optimize, and snapshot witness generation

@benoitrazet

There are likely parts of the risc-v emulation that is not being used. Clean this up and remove unused parts.
Run benchmarks, flamegraphs, etc for risc-v emulation and optimize any low hanging fruit here. Potentially follow the Succinct's RISC-V Emulator Challenge to see if there are any ideas we can use here.
Implement snapshotting of trace generation. During the first run, snapshot the CPU state and memory every N steps. Then add the ability to rerun the emulation from a snapshot, producing a stream of the trace. (this task should likely be in a separate PR)

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