Add splitAt function to Cryptol book

[marsella]

The splitAt function isn't in the book!

> :h splitAt 

    splitAt : {front, back, a}
      (fin front) =>
      [front + back]a -> ([front]a, [back]a)

Splits a sequence into a pair of sequences.
'splitAt z = (x, y)' iff 'x # y = z'.

We should add it, at least to Appendix B (the cryptol prelude description).

[RyanGlScott]

Interestingly, there is a commented-out section at the end of the book (here) that includes splitAt as well as some other functions. After scavenging splitAt out of this commented-out section, we should see if any of the other functions in that section also need to be mentioned in the book.

[yav]

When we update the book it might make sense to also point out the relation between splitAt and # patterns.