minimongo: Save all array indices, not just first

This 'x' thing is a bit of a hack. The idea is that there are two
consumers of arrayIndices. One is '$', which only looks at the first
index. The other is (soon) sort ken generators, which cares if indices
are explicit or implicit, but doesn't really need to "parse"
arrayIndices; it just will use them to associate branches from different
lookup functions with the matching arrayIndices.  'x' is a simple way to
accomplish this, though I could have made the values of arrayIndices be
objects too.
Failing test-case for a sort bug