manifest.c: new extension code to lazily parse manifests This lets us iterate manifests in order, but do a _lot_ less work in the common case when we only care about a few manifest entries. Many thanks to Mike Edgar for reviewing this in advance of it going out to the list, which caught many things I missed. This version of the patch includes C89 fixes from Sean Farley and many correctness/efficiency cleanups from Martin von Zweigbergk. Thanks to both!