Add TraceableStack object and use it for the colocation stack to improve...

Add TraceableStack object and use it for the colocation stack to improve (future) error messages.  The TraceableStack directly manages TraceableObjects.

The TraceableStack object acts as a regular stack for arbitrary objects.  Its object-encapsulation class, TraceableObject, includes metadata fields including an arbitrary name, and members and methods to track the filenames and line numbers of callers of the stack's call push_obj().  The contents of a TraceableStack can easily be summarized by a dict mapping the TraceableObject.name to new TraceableObjects that contain only the name, file, and line number metadata.  This is useful for constructing public return values.

PiperOrigin-RevId: 204288464