Use object-based elements within SortFieldWriter.

Use objects rather than structs as an elements within SortFieldWriter,
harmonizing with all other subclasses of SortExternal.
Modify SortEx_Feed() argument type.

Have Feed() take an Obj* rather than void*.
Change return type for Peek() and Fetch().

Modify SortExternal and subclasses so that Peek() and Fetch return Obj*
rather than a void* which was actually Obj**.
Change type of SortEx members.

Change slice_starts and scratch to limit element types to Obj.