Since Tree.meta is property with additional presence check it
introduces unnecessary overhead in accessing underlying Meta
object once it's already initialized. Having local reference to actual
Meta object allows to reduce these presence checks to bare minimum

Using OrderedDict provides these properties:
- given subtree is yielded only once since it's stored in OrderedDict
only once (even though it may be put there multiple times) so no need
to double check if subtree was already seen
- order of iteration is preserved as it was previously when subtrees
to iterate over were stored in a list