change required top level object to be either array or object type

[b5]

For a while now I've been rather insistent that the top level of a dataset be an array. I've now come across a number of examples in the wild where having an object as the top level is just plain necessary. For example:

data.json catalogs
top level is an object. It's a dataset. qri should get over it.

site maps
Doing de-duplicated sitemaps is much easier using normalized urls as keys in an object.

So based on this, I think we need to relax the definition to say that a top level dataset needs to be either an array or an object. This'll affect a few things:

• dsio.Reader will need to recognize object keys as entries
• any validation needs to be cool with { "type": "array" } or {"type":"object"} as a root schema

I'll add to that list as things go.