Implement clojure.core/shuffle

[bpiel]

Closes #220

clojure.core=> (shuffle [3 2 1 4 5 6])
[5 6 3 2 4 1]
clojure.core=> (shuffle [3 2 1 4 5 6])
[4 5 3 1 2 6]
clojure.core=> (shuffle '(1 2 3 4 5))
[1 3 5 2 4]
clojure.core=> (shuffle {1 2 3 4 5 6})
[[1 2] [5 6] [3 4]]
clojure.core=> (shuffle #{1 2 3 4 5 6})
[2 4 3 1 6 5]

NOTE: swap was renamed to swap_atom, because otherwise std::shuffle was calling it and expecting typical C++ swap behavior.

[jeaye]

Looks good, with a couple minor comments.

[jeaye]

Nice work!