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.

apidoc/ostree-sections.txt

@@ -151,6 +151,7 @@ ostree_validate_structureof_dirmeta
 ostree_commit_get_parent
 ostree_commit_get_timestamp
 ostree_commit_get_content_checksum
+ostree_commit_get_object_sizes
 OstreeCommitSizesEntry
 ostree_commit_sizes_entry_new
 ostree_commit_sizes_entry_copy