struct/class consistency

Summary:
This diff is based on https://github.com/facebook/folly/pull/95
by gitbug user mikekap.

It changes the struct/class naming of forward declarations to match
their definitions.

Test Plan:
fbconfig -r folly && fbmake runtests

Reviewed By: markisaa@fb.com

Subscribers: fugalh, folly-diffs@, jsedgwick, yfeldblum, chalfant

FB internal diff: D2032995

Tasks: 6783581

Signature: t1:2032995:1430339337:79193fd8409b73e8a9155133f326b7cee534244d

folly/MPMCQueue.h

@@ -34,7 +34,7 @@ namespace folly {
 namespace detail {
 
 template<typename T, template<typename> class Atom>
-class SingleElementQueue;
+struct SingleElementQueue;
 
 template <typename T> class MPMCPipelineStageImpl;
 

folly/wangle/rx/Observer.h

@@ -24,7 +24,7 @@
 
 namespace folly { namespace wangle {
 
-template <class T> class FunctionObserver;
+template <class T> struct FunctionObserver;
 
 /// Observer interface. You can subclass it, or you can just use create()
 /// to use std::functions.

folly/wangle/rx/types.h

@@ -25,7 +25,7 @@ namespace folly { namespace wangle {
   // alias it.
   typedef std::shared_ptr<folly::Executor> SchedulerPtr;
 
-  template <class T, size_t InlineObservers = 3> struct Observable;
+  template <class T, size_t InlineObservers = 3> class Observable;
   template <class T> struct Observer;
   template <class T> struct Subject;