Add operator== and operator!= to folly::StlAllocator

Summary: Add operator== and operator!= to folly::StlAllocator, which are required by some other libraries.

Test Plan: Tested by running with recursive_wrapper in boost::variant, and unit tests in folly/test.

Reviewed By: delong.j@fb.com

FB internal diff: D541237

folly/StlAllocator.h

@@ -116,6 +116,14 @@ class StlAllocator {
     typedef StlAllocator<Alloc, U> other;
   };
 
+  bool operator!=(const StlAllocator<Alloc, T>& other) const {
+    return alloc_ != other.alloc_;
+  }
+
+  bool operator==(const StlAllocator<Alloc, T>& other) const {
+    return alloc_ == other.alloc_;
+  }
+
  private:
   Alloc* alloc_;
 };