Summary: The pattern `mapped([](auto&& i) { ...; return std::move(i); }` is quite common. Evidently people need some good mechanism to just observe or possibly mutate items in pipelines. Note that this is still lazy, it is only called when the pipeline is run.

Reviewed By: yfeldblum

Differential Revision: D5457196

fbshipit-source-id: 3a892b8895e02dd8fcd6a7fd4d3b27063d1b071f