Reimplement `miden repl`

#2483 removed the `VmStateIterator`, which `miden repl` depended on (and hence removed that subcommand). We should reimplement that command using the `FastProcessor`, though further discussion would be required for exactly what we want from that subcommand.

Further context: https://github.com/0xMiden/miden-vm/pull/2483#discussion_r2635302376

Look at the removed VmStateIterator in PR #2483 and the linked discussion to understand the old repl design. The new implementation must use the FastProcessor. Start by examining the miden-vm codebase for the repl command structure and the FastProcessor's public interface. Determine what the repl should do: likely step through VM execution and inspect state. Check for existing tests or examples of FastProcessor usage.

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