[SetTheoretic] Minimal Representation via Accessors

Consider the following minimal representation for `SetTheoretical`:

Refines `hana::Foldable` (unless we want infinite sets???)

Implements `get(s)` where T is like a key for an element.

Implements `key_of(e)` where `e` is an element belonging to a set of tag `S`

Implements `make` (or both `insert` and `erase_key`)

Then implementations of the currently proposed `SetTheoretical` concept such as `union_`, `intersection`, etc. would be provided by the above.

For `Searchable` the only issue I still see is with providing an efficient `contains` from `get`. Perhaps if it was somehow SFINAE friendly or it returned an optional or something.

Start by reading the existing SetTheoretical and Searchable concept definitions, then compare them with the proposed get, key_of, make, insert, and erase_key accessors. Done would require an agreed minimal representation design and corresponding updates to the affected concepts and operations; no files or tests are named in the issue.

Written by the indexing model from the issue text.