The new DWARF indexer

The new DWARF indexer

This patch adds the code to index DWARF.  This is just the scanner; it
reads the DWARF and constructs the index, but nothing calls it yet.

The indexer is split into two parts: a storage object and an indexer
object.  This is done to support the parallelization of this code -- a
future patch will create a single storage object per thread.