make R.times throw if given an invalid array length

This is rather fitting, given the strange behaviour was highlighted by a remark in a thread celebrating 1000 stars:

<img alt="1000 passing" src="https://cloud.githubusercontent.com/assets/210406/5174016/77adf184-73e2-11e4-8e79-8080c0eacfe0.png" width="224" height="56">