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-core-private.h

@@ -102,6 +102,9 @@ _ostree_checksum_inplace_from_bytes_v (GVariant *csum_v, char *buf)
  */
 #define _OSTREE_LOOSE_PATH_MAX (256)
 
+/* GVariant format for ostree.sizes metadata entries. */
+#define _OSTREE_OBJECT_SIZES_ENTRY_SIGNATURE "ay"
+
 char *
 _ostree_get_relative_object_path (const char        *checksum,
                                   OstreeObjectType   type,