LXML Template Unfication

This DSL provides a light-weight way to extract fields from nested
HTML/XML structure represented in LHTML (as produced by closure-html).
A template is a declarative representation of substructure with
embedded variables that are bound when the substructure matches.

Substructure is loosely matched, such that if any given body element
doesn't match, the next child is considered until all the template
body elements have matched a lhtml element or the end of the elment
has been reached without a match.

Prepending < to a tag enables a depth-first search for that tag so you
can avoid specifying the parent path (similar to // in xpath)

Any matching template that consists of a variable reference results in
a binding set being created and returned if all elements of the
template node successfully match.

Additional reserved operators allow you more flexibility on 
managing what is matched and how bindings from subtrees are combined

all: match same template multiple times over the children of a given node
     and store them as a list attached to a fresh bindings list

merge: create a single binding out of each of the sub-bindings.  A 
     node body has an implicit merge

nth: find the nth instance that matches the full body of this operator

regex: matches if regex returns register values for a string (as a list)

fn: Run the referenced function symbol on the current parse state and
    return bindings, t or nil as appropriate.