[DSLX] Feature: Proc network evaluator that uses `ProcRuntime` interface

DSLX doesn't have a common object to handle simulation.
https://github.com/google/xls/blob/44b271abe4d1f310ed08ebacaa6198d9e6ee2393/xls/dslx/run_routines.cc#L75-L127
defines process evaluation loop, but it is only available when running `#[test_proc]`.

It would be useful to have an object that uses the same `ProcRuntime` interface as IR presses do.
I think it would make XLS API more robust, as it would unify IR and DSLX simulation API.
It would make swapping between IR/DSLX frontends seamless.