Start moving the sandbox out of detail/
Fix ambiguities when defining stuff in the operators namespace
Sandbox: add SearchableSet
Add local todos.
Add the Searchable type class.

Remove any, all, none, find, elem and the *_of variants from Foldable
and move them into Searchable.
Maybe: instantiate Searchable
List: instantiate Searchable
Foldable: Remove lazy_foldr and lazy_foldl
Searchable: add the find_key method