Add an option to parse-string that maps YAML sequences to Clojure vectors instead of Clojure LazySeqs

[bombaywalla]

It would be useful to have parse-string return a Clojure Vector rather than a Clojure LazySeq for YAML sequences.
It would allow the use of get-in on the resultant Clojure data structure.
io.forward/yaml does this.

[borkdude]

This has come up before here: #18
The reason why this wasn't done was to support recursive YAML (which arguably is a niche case).

[bombaywalla]

Sorry. I should have checked closed issues before creating a new one.
I'll use the postwalk suggestion mentioned in the previous issue.
Closing this issue.