Support for different models of brace alignment
Helper function for matching children within trees.

Probably mostly will be used internally, but it's potentially more
generally useful.  But mostly not.
Update tree_style to use tree_match internally.

A few other changes came along for the ride here, including
fixing an inconsistent pluralisation of tree_label/tree_labels.
Pluralise tree_brace -> tree_braces everywhere
Support non-nested styling (single nodes)