Fix parser bug with sequential selection sets

[ilmoraunio]

It was possible to write:

(oksa.core/gql [[:foo {} [:qux :baz]] [:basho]])
; or
(oksa.core/gql
  (oksa.alpha.api/select (oksa.alpha.api/field :foo
                                (oksa.alpha.api/select :qux :baz))
                         (oksa.alpha.api/select :basho)))

resulting in incorrect GraphQL:

{foo{qux baz}{basho}}

This PR fixes the parser to consider sequential selection sets as invalid. We also remove an API test case that was invalid to begin with. (Edit: removed, see explanation.)

[ilmoraunio]

Well, apparently the removed test did make some sense after all. You can have a special case when you can have a (non-naked) field without a containing selection set + a subsequent selection set.

Case in point:

(unparse-and-validate [[:foo {}] [:bar]])
; or
(unparse-and-validate
               (api/select
                 (api/field :foo)
                 (api/select :bar)))
; => "{foo{bar}}"

I've made some changes to accommodate this.