Performance issue of generators with spec.alpha/merge

I've been trying to debug some performance issues with my tests and have identified `spell-spec` as an unexpected culprit. The issue is definitely noticeable when generating examples with `spec.alpha/merge`, but I have not investigated the root cause.

Test case:

```
(tufte/add-basic-println-handler! {})

(s/def ::foo pos-int?)
(s/def ::bar neg-int?)
(s/def ::baz string?)
(s/def ::qux keyword?)

(deftest speed-difference-test
(let [spec1 (s/merge (s/keys :req [::foo])
(s/keys :req [::bar])
(s/keys :req [::baz])
(s/keys :req [::qux]))
spec2 (s/merge (spell-spec/keys :req [::foo])
(spell-spec/keys :req [::bar])
(spell-spec/keys :req [::baz])
(spell-spec/keys :req [::qux]))]
(tufte/profile
{}
(tufte/p :spec (doall (take 100 (gen/sample-seq (s/gen (s/spec spec1))))))
(tufte/p :spell (doall (take 100 (gen/sample-seq (s/gen (s/spec spec2)))))))))
```

Result:

```
pId nCalls Min 50% ≤ 90% ≤ 95% ≤ 99% ≤ Max Mean MAD Total Clock

:spell 1 909.46ms 909.46ms 909.46ms 909.46ms 909.46ms 909.46ms 909.46ms ±0% 909.46ms 97%
:spec 1 27.39ms 27.39ms 27.39ms 27.39ms 27.39ms 27.39ms 27.39ms ±0% 27.39ms 3%
```

Tested with:

```
[com.bhauman/spell-spec "0.1.1"]
[org.clojure/spec.alpha "0.1.143"]
[org.clojure/clojure "1.9.0"]
```

Start by running the provided speed-difference-test with the listed Clojure, spec.alpha, and spell-spec versions, comparing spell-spec/keys with spec.alpha/merge under tufte/profile. Trace the generator path used by s/gen and gen/sample-seq to find where the slowdown occurs. Done means identifying the regression cause, improving the performance, and preserving coverage for the reported comparison.

Written by the indexing model from the issue text.