renamespace bits of folly/compression/ that are in folly::io

Summary:
Renamespace these bits to `folly::compression` from `folly::io`, but leave aliases in place in `folly::io` for now.
Fixes: facebook/folly#700.

Differential Revision: D58819499

fbshipit-source-id: 5270a4bde3c0f6ba55d02e96d8d21070afc8e2fb

proxygen/lib/utils/ZstdStreamCompressor.h

@@ -15,9 +15,12 @@
 
 namespace folly {
 class IOBuf;
-namespace io {
+namespace compression {
 class StreamCodec;
 }
+namespace io {
+using folly::compression::StreamCodec;
+}
 } // namespace folly
 
 namespace proxygen {