core: Add ostree_commit_get_object_sizes API

This function parses the object listing in the `ostree.sizes` metadata
and returns an array of `OstreeCommitSizesEntry` structures.

Unfortunately, for reasons I don't understand, the linker wants to
resolve `_ostree_read_varuint64` from `ostree-core.c` even though it's
not used by `test-checksum.c` at all.

src/libostree/ostree-repo-private.h

@@ -31,8 +31,6 @@ G_BEGIN_DECLS
 
 #define OSTREE_DELTAPART_VERSION (0)
 
-#define _OSTREE_OBJECT_SIZES_ENTRY_SIGNATURE "ay"
-
 #define _OSTREE_SUMMARY_CACHE_DIR "summaries"
 #define _OSTREE_CACHE_DIR "cache"