Help the user transition to a streaming program

[gelisam]

The documentation guides the user from one liners to a Haskell program, but there is a missing step: hawk processes the input into a table and streams the expression through the user's function, but a Haskell program doesn't. Worse, that functionality is in a part of hawk which is marked as internal and unstable, so the user will have to reimplement it themselves!

We could expose that part as a more stable library and explain how to turn a hawk prelude file into a Haskell project which uses it. Or, even better, add a hawk command which generates this project from a hawk prelude file and an expression! At that point, we could either copy the streaming code into the generated project, which would allow the user to tweak it, or generate a project which refers to a hawk library, which would allow the user to benefit from future bugfixes to that library.