The old describe-instances is deprecated, and the new describe-instances no
longer returns a DescribeInstancesResult.  describe-instances now always
returns a seq of Instance objects.  This also fixes paging support so we
return all instances.  The new describe-instances supports all filters per
the API, only limits filters to one Filter object per key.

:region support is eliminated for report-instances, you can no longer use
it to get instances from all regions via report instances, do your own
describe-instances calls per region if you want it.

describe-instances uses a new keyword validation routine we should put in
any other routines with complicated keyword sets, to avoid mistyped/invalid keywords.