Reconsider non-scalar comparisons

[glyn]

For example, running the selector:

$[?(@[*] > 3)]

against the document:

[[1,2],[3,4],[5,6]]

produces the result consisting of a single entry:

[5, 6]

Some implementations, such as Proposal A, fail to parse non-scalar comparisons.

[glyn]

See #3 (comment) for a bit of theory about this. Keeping this behaviour for now unless and until a standard says otherwise.