Maps as keys are not fully supported

[stathissideris]

> (pprint-specs
    (infer-specs [{{:a 4} 3}
                  {{:a 8} 4}] ::foo) *ns* 's)

(s/def ::foo (s/map-of (s/keys :req-un [::a]) integer?))

::a should have been promoted to top-level as a named spec