Take pry out of the dev dependencies
spike on runner/pipeline
Get the Runner to a working state

Yaks::Runner generalizes the concept of a pipeline of transformations,
so it's easier to provide before/after/around hooks for each steps.

Also contains a fix for passing in an item_mapper to Yaks#call.
Add another benchmark

This is just the first, the idea is to compare flat vs deep structures.
Prevent freezing the Rack env

Switch to flat freezing in Runner, because it otherwise would deep
freeze the whole rack env.