pod: improve parser and builder

Remove the spa_pod_iter helpers
Remove builder/parser vararg recurse option, you have to
manually recurse into structures when needed. This simplifies
things a lot.
Pass spa_pod_frames to builder and parser explicitly, we don't
have to keep an internal stack anymore.
The parser is now almost a mirror image of the builder.
Make the parser safer when iterating over objects, add functions
to check and get pod contents in a safe way.
Make the builder return errno style results on errors
Improve performance of object properties when they are stored and
retrieved in the same order.
Add many more tests for the builder and parser
Add some benchmarks

Makefile.in

@@ -25,6 +25,9 @@ valgrind:
 test: all
 	ninja -C build test
 
+benchmark: all
+	ninja -C build benchmark
+
 monitor: all
 	SPA_PLUGIN_DIR=build/spa/plugins \
 	PIPEWIRE_MODULE_DIR=build/src/modules/ \