Renaming and cleanup in the processor modules

After https://github.com/0xMiden/miden-vm/pull/2651 and https://github.com/0xMiden/miden-vm/pull/2681 processor modules will be in a pretty good shape. A few remaining this are:

- As mentioned in https://github.com/0xMiden/miden-vm/issues/2495#issuecomment-3880108782, rename `FastProcessor` into just `Processor`. This will require renaming the current `Processor` trait into something else. @plafer suggested `AbstractProcessor`. Another option could be `Executor`.
- Make sure we export all the relevant components from the processor. Could be done after https://github.com/0xMiden/miden-vm/pull/2683 - though, we need to be a bit more surgical here.
- I'd love to clean up the internal organization of the `trace` module in the processor (e.g., do we still need `parallel` namespace since this is now the only way to generate traces?)
- Should we switch to using `maybe_rayon` from P3 for parallel operations? (though, this is probably a separate issue).
- We should re-visit the top level `execute()` function. Right now, we don't really use it in meaningful ways. We could either remove it (and always require using `Processor` directly) - or we could start using it instead of using the processor directly. If we do keep the function, I'd make the top level exports to be `execute()` that doesn't build the trace and `build_trace()` that executes and builds the trace.