Ensure that (:streams core) is always a vec.

Clojure.core/concat always returns a LazySeq, not a vec, and LazySeq has some
synchronized badness (see src/jvm/clojure/lang/LazySeq.java, line 59).
Merge pull request #179 from banjiewen/vector-streams

Ensure that (:streams core) is always a vec.