ID linked to type information

[ichthyo]

At various places, we attach additional (static, compile-type) type information with an ID type.
The purpose is to pass on a specifically typed evaluation context, allowing to use more dedicated APIs and avoid the design problems arising notoriously when we make "everything an object".

The really interesting question here is: is there a way to map the relations between target types as well, and is it even desirable to do so? (recall, for containers the answer is "no" -- it would subvert type safety. To the opposite, smat-ptrs often do it in some limited way, by recurring on the underlying type, see e.g. static_pointer_cast).

It's certainly not necessary to investigate this problem immediately to get ahead, but it should be sorted out eventually...