Write an F# function shuffle xs that takes an even-length list, cuts it into two equal-sized pieces, and then interleaves the pieces:

 > shuffle [1;2;3;4;5;6;7;8];; val it : int list = [1; 5; 2; 6; 3; 7; 4; 8] 

(On a deck of cards, this is called a perfect out-shuffle.)