This replaces the simple string used by pa_format_info's proplist with a
JSON string (accessed via new API only). This allows us to express lists
and ranges more cleanly, and embed type information for future
extensibility.
We use json-c for JSON parsing. This is a lightweight depdency (32 KB on
my system) and avoids the hassle of having to reinvent a JSON parser.
Also included is a test which verifies functionality and is
valgrind-clean.
