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/libostree-devel.sym

@@ -20,6 +20,7 @@
 /* Add new symbols here.  Release commits should copy this section into -released.sym. */
 LIBOSTREE_2019.7 {
 global:
+  ostree_commit_get_object_sizes;
   ostree_commit_sizes_entry_copy;
   ostree_commit_sizes_entry_free;
   ostree_commit_sizes_entry_get_type;