Cleanup, optimize, and snapshot witness generation

[jprider63]

1. There are likely parts of the risc-v emulation that is not being used. Clean this up and remove unused parts.
2. 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.
3. 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)

@benoitrazet @zutshi

[moodlezoup]

Adding some color here:

1. There are likely parts of the risc-v emulation that is not being used. Clean this up and remove unused parts.

In particular, I think a lot of the stuff in tracer/src/emulator/device/ is unused, as none of those devices exist in the context of a zkVM guest program

2. 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.

Similar to what we do in jolt-core, I'd suggest instrumenting the code with tracing decorators and spans, writing some benchmarks (including some longer execution traces, i.e. >2^24 steps), and analyzing the resulting flamegraphs in Perfetto.

3. 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)

I envision modifying the trace function to have the following signature:

pub fn trace(
    elf: &PathBuf,
    inputs: &[u8],
    input_size: u64,
    output_size: u64,
    checkpoint_interval: Option<usize>
) -> (Vec<RVTraceRow>, JoltDevice, Option<Vec<TraceCheckpoints>>)

and introducing a new function that looks roughly like this:

pub fn trace_from_checkpoint(checkpoint: TraceCheckpoints) -> impl Iterator<Item = RVTraceRow>