Support only consuming the input iterator of iterators when space in the thread pool frees up

[jwodder]

• Implement this with a function named interleave_as_needed() or similar

  • Name the corresponding submit() variant submit_as_needed()

• Allowing users to submit iterators normally to this sort of interleaver seems likely to cause problems; it may be necessary to return a variant (LazyInterleaver?) interleaver class (inheriting from a base class) that doesn't have public submit() and finalize() methods

• Don't forget to handle errors raised while iterating over the super-iterator!

  • Make interleave react the same way it does to errors in the threads?