Remove @/folly dependency from folly/io:compression

Summary: This diff should let us include folly/io:compression into iOS and Android projects without pulling in a lot of dependencies and should allow the use of zlib.

Test Plan: fbconfig -r folly/io/tests && fbmake runtests

Reviewed By: seanc@fb.com

Subscribers: trunkagent, bmatheny, folly-diffs@, yfeldblum, seanc, benyluo

FB internal diff: D1853058

Tasks: 6245912

Signature: t1:1853058:1424370881:2ea243d93e6041502e5e356fa430c2483f652b40

folly/Varint.h

@@ -18,6 +18,7 @@
 #define FOLLY_VARINT_H_
 
 #include <type_traits>
+#include <folly/Conv.h>
 #include <folly/Range.h>
 
 namespace folly {
@@ -126,7 +127,8 @@ inline uint64_t decodeVarint(Range<T*>& data) {
     }
     if (p == end) {
       throw std::invalid_argument("Invalid varint value. Too small: " +
-                                  std::to_string(end - begin) + " bytes");
+                                  folly::to<std::string>(end - begin) +
+                                  " bytes");
     }
     val |= static_cast<uint64_t>(*p++) << shift;
   }

folly/io/Compression.cpp

@@ -915,9 +915,12 @@ std::unique_ptr<IOBuf> LZMA2Codec::doUncompress(const IOBuf* data,
 
 #endif  // FOLLY_HAVE_LIBLZMA
 
-typedef std::unique_ptr<Codec> (*CodecFactory)(int, CodecType);
+}  // namespace
+
+std::unique_ptr<Codec> getCodec(CodecType type, int level) {
+  typedef std::unique_ptr<Codec> (*CodecFactory)(int, CodecType);
 
-CodecFactory gCodecFactories[
+  static CodecFactory codecFactories[
     static_cast<size_t>(CodecType::NUM_CODEC_TYPES)] = {
     nullptr,  // USER_DEFINED
     NoCompressionCodec::create,
@@ -953,17 +956,14 @@ CodecFactory gCodecFactories[
     nullptr,
     nullptr,
 #endif
-};
-
-}  // namespace
+  };
 
-std::unique_ptr<Codec> getCodec(CodecType type, int level) {
   size_t idx = static_cast<size_t>(type);
   if (idx >= static_cast<size_t>(CodecType::NUM_CODEC_TYPES)) {
     throw std::invalid_argument(to<std::string>(
         "Compression type ", idx, " not supported"));
   }
-  auto factory = gCodecFactories[idx];
+  auto factory = codecFactories[idx];
   if (!factory) {
     throw std::invalid_argument(to<std::string>(
         "Compression type ", idx, " not supported"));