Avoid repeated traversal of tree in History construction #1139

The History objects used in the new H-selector construction API are rebuilt recursively with no sharing between calls - so in effect selection is exponential when it needs only to be linear (in the depth of the tree).

The crux of this is that _history_of(node) walks back to the root and calls _canonical_history to fill in the history transition - but _canonical_history calls history_of again.

The proposed fix is that canonical-history construction should be memoised.