[DEPRECATION] UnManagedStruct handling nested structure

[plobsing]

UnManagedStruct is complicated by the need to handle nested structures. The need to consult nested structures for their shape, support for multi-element keyed lookup, etc...

These are a higher level concepts than "view this buffer as a collection of these datatypes", and as such do not belong in UnManagedStruct. Therefore the following aspects of unmanagedstruct are deprecated:

• remembering structure of nested structured elements. These will now be treated as void * or void (*)(void) as appropriate
• multi-level keyed access