Summary:
[Folly] Move `FOLLY_DEFINE_CPO` to `folly/lang/CustomizationPoint.h`.

And move `StaticConst` to top-level `folly` namespace and into `folly/lang/`.

And Americanize the spellings of words in the comments.

Reviewed By: ericniebler, lewissbaker

Differential Revision: D14402315

fbshipit-source-id: e13dbc9d471ee73ba0a289d375c235734334b920