Namespace rename from UpperCamelCase to snake_case

Namespace rename from UpperCamelCase to snake_case

Summary:
`folly::literals` is to `folly` what `std::literals` is to `std`.

More importantly we should be consitent;  snake_case is how other folly sub-namespaces are named, such as:

`array_detail`
`apply_tuple`
`format_value`
`threadlocal_detail`
`recordio_helpers`
`exception_tracer`

Reviewed By: ericniebler

Differential Revision: D4468281

fbshipit-source-id: de10bec74c9a38f387d62bd55da947291c23970b