Compression namespace

[RcRonco]

Hi,
When compression moved from io to its own folder the namespace still stayed folly::io, which is quite confusing.
Maybe it should be in its own namespace or in the folly namespace, or even return the compression into io folder.

In compression Utils.h the namespace folly::io::compression exists, refactoring the namespaces of the compression folder will make it more consistent, I don`t mind to do it.

[Orvid]

I believe the plan is to shift the namespace as well, we're just doing all of the physical moves first, then we'll be renaming a few things to make them more consistent (this included), and finally post a write-up of everything that moved and where it went :)