Publicize type constants

We are going to need to return these after all, so we'll have to export them in
the installed header. We also need to rename them to give them a namespace-safe
prefix.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>
Add iterator API

I've been putting this off for too long. We now have an API to traverse through
ASON values and pull values out as C values. The utility is self-evident.

Signed-off-by: Casey Dahlin <cdahlin@redhat.com>