STAMQL: Allow multiple subqueries

[proycon]

Currently a query only allows a single subquery. This was done to ensure all
variables in the results are always bound.

Having multiple subqueries means a combinatorial 'explosion' in the result set.
Given a query (A) and subqueries (X,Y,Z) where X in turn has subqueries (C,D).
We have the following hierarchy:

?A
    ?X
        ?C
        ?D
    ?Y
    ?Z

The result row binds not just a single set of variables now but differs according to the subquery path:

?A * ?X * ?C
?A * ?X * ?D
?A * ?Y
?A * ?Z

Say ?A yields a,b,c and ?Z yields p,q, then for the last bindings alone we get six results:

a p
a q
b p
b q
c p
c q

In spite of the combinatorial explosion, having multiple subqueries may give
necessary flexibility. Most notably, I think we can do away with the concept of
'highlight queries' in visualisation then, as subqueries can take over that role.