Parser allows sequential fragment spread and selection set

[ilmoraunio]

Another finding with generative tests, using ./bin/run-gen-tests --seed 1133573659.

Invalid syntax with offending token '{' at line 1 column 17

This exception is coming from an external GraphQL parser, so we are producing incorrect GraphQL here.

Reduced example:

(gql '[:oksa/mutation [[:... {:name :f}] [["E1lgzB" {}]]]])
=> "mutation {...f{E1lgzB}}"

We should restrict the parser not to allow this corner case.

Related: #9