one separate library for answer tests

So it’s easier to run separately, and so we can test by shelling out to euler, which means we can run in parallel.

Alternatively, figure out how to make dune run tests in parallel.

`Solution_intf.Arg.main` should return a string

Might be easier to automate things if it returns a string instead of printing.

let () = unimplemented [%here]

E.g., using ppx_bench. This might require build system support.