Enforce rejecting top-level arrays

[colmsnowplow]

In our documentation we express a firm position that for self-describing data, the top-level field containing the data must be an object. In other words, we can't define an event that contains an array at the top level, as follows:

var contextsString = `{"schema":"iglu:com.snowplowanalytics.snowplow/contexts/jsonschema/1-0-0","data":[{"schema":"iglu:com.acme.test/testing/jsonschema/1-0-0","data":["aa","bb"]}]}

I think this is a sensbile restriction to impose, and I think it should be enforced at validation, since downstream tooling shouldn't have to accommodate the possibility. We ran into this issue, for example, in the go analytics sdk: snowplow/snowplow-golang-analytics-sdk#41