Summary:
For the initial cutover, just pull the copies in folly into the wangle namespace
The main problem with that approach is that forward declarations of wangle components no longer work, so I fixed those manually

ALSO, IMPORTANT: This is a great, once in a lifetime opportunity to rename/restructure these components. I have a few ideas that I'll noodle and share eventually, e.g. changing LifoSemMPMCQueue so it's just descriptive and not an enumeration of implementation details. Please chip in with yr ideas!

Reviewed By: yfeldblum

Differential Revision: D5694213

fbshipit-source-id: 4fc0ea9359d1216191676fc9729fb53a3f06339f