A terminal node for a query. Holds the tokens that reached it, to be read back by name.
:params is the head of the query. These are the bindings that key its tokens, and
they are the only way to read it. Rete.Engine.query/3 takes exactly these, so a read
is one map lookup. An empty list is a query with no parameters. Its tokens all key on
%{}, and they come back in arrival order.