Rethink STS.

[kohlhase]

The STS type system could be rethought. There are a multitude of issues

• [ ] CD symbols as basic STS Types (see https://github.com/OpenMath/OM3/issues/27), this also includes the notion of a new role for types (see https://github.com/OpenMath/OM3/issues/28).
• [ ] there is a fundamental redesign proposal by @lars-hellstrom (see https://github.com/OpenMath/OM3/issues/152).
• [ ] there is a clarity issue about binders. (see https://github.com/OpenMath/OM3/issues/151)
• [ ] in any case, the STS document, is under standard, should it really have this elevated status?