organization of this directory is moving towards this approach:

<module>.h               -- public API and data types
<module>_private.h       -- protected API and data types
<module>.c               -- implementation and private API / types

The rules are:
.c files may include *_private.h.
.h files may not include *_private.h

This allows us to have a nicely encapsulated C api with opaque data types and private functions
that are nonetheless shared between source files without redundant extern declarations..