`FastProcessor::execute` should return a list of checkpoints

[plafer]

This is part of the parallel execution line of work #1558.

This list of "checkpoints" is all the information that is needed by a trace generator to generate a trace. This is to be used by the TransactionProver in miden-base, as well as by the "trace fragment generators" in #1839.

[bobbinth]

Overall, FastProcessor::execute() should ideally return 3 things:

• SackOutputs - as it does now.
• AdviceProvider - this is needed build objects that are committed to by the stack outputs (i.e., stack outputs contain commitment to some data, but the actual data is in the advice provider).
• Checkpoints - this is what this issue is about. These checkpoints should be serializable because we frequently would send them to a remote prover for actual proof generation.