Simplify parameter handling with map

Right now managing the parameters of the `iterator` to a `map` is challenging, because there's no way to use the input schema to ensure you're passing things to the right places.

It'd be great if the API provided some facility for doing this to reduce the difficulty.

Start by reading the iterator and map parameter-handling entry points, along with how the input schema is represented and consumed. Define how schema-driven parameter mapping should work and what validation or failure behavior is expected; the issue does not name specific files or tests.

Written by the indexing model from the issue text.