Improve error messages

[danielcompton]

Currently the error messages returned by iota have gensyms and other internals which makes it harder to see the real test failure. For example:

clojure.test

expected: ["45"]
  actual: #{"15" "30" "45"}

iota

expected: (clojure.core/=
           ((juxt.iota/as-test-function
             [:body :keen.query/response :workspace/duration-ids])
            G__53963)
           #{"45"})
  actual: (not (clojure.core/= #{"15" "30" "45"} #{"45"}))

It might also be interesting to be able to show multiple failures for a single given in a single message, similar to https://github.com/facundoolano/restpect#test-reporter.