modularize example state machine code for generic usage

[chung-thai-nguyen]

[x] Move existing VM-related structs into a separate file for quick usage at zkmemory/src/default_state_machine.rs

[chiro-hiro]

Thanks for the refactor — extracting StandardStateMachine/StandardInstruction into default_state_machine.rs is a good structural improvement and the $crate macro fix is a real correctness win.

A few things to address before merging:

1. Dead RNG call (randomize.gen_range(u64::MAX / 2..u64::MAX) with the result discarded) appears in both the test in default_state_machine.rs and in 256bits-machine.rs. Remove the line or assign it if you intend to use it.

2. Typo in example output: "Intruction: {:?}" → "Instruction: {:?}" in 256bits-machine.rs.

3. memory-consistency example removed with no replacement: renaming examples/memory-consistency.rs to a library source file removes a runnable example. If this was intentional, please add a note to the README and check any CI jobs that ran cargo run --example memory-consistency.

4. assert_eq!(sections.len(), 3) is always true at compile time. Not a bug, but it adds no test value.