Move Singleton out of folly/experimental into folly/

Move Singleton out of folly/experimental into folly/

Test Plan: contbuild

Reviewed By: chip@fb.com

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FB internal diff: D2012267

Tasks: 5676394

Signature: t1:2012267:1430334667:eaad0262b35ffbfae86df5bdb45bf057ac62c51b