Move hash_128_to_64 to top of file
Move hash_128_to_64 to top of file

Summary: [Folly] Move `hash_128_to_64` to top of file - it has no dependencies so there was no need to split the declaration and definition across 500 lines.

Reviewed By: Orvid

Differential Revision: D17276906

fbshipit-source-id: f57133e8873d8b1afcfd5ab119e07615f6ee9018

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly