(wangle) Fix a couple compilation issues

Summary: If you try to include files in a weird (but not incorrect) order you get compilation errors. No longer!

Test Plan: new dummy `cpp_binary` target

Reviewed By: davejwatson@fb.com

Subscribers: trunkagent, folly-diffs@, net-systems@, fugalh, exa, njormrod

FB internal diff: D1621083

folly/wangle/Future.h

@@ -29,6 +29,12 @@
 
 namespace folly { namespace wangle {
 
+namespace detail {
+  template <class> struct Core;
+  template <class...> struct VariadicContext;
+}
+template <class> struct Promise;
+
 template <typename T> struct isFuture;
 
 template <class T>

folly/wangle/Promise.h

@@ -17,7 +17,6 @@
 #pragma once
 
 #include <folly/wangle/Try.h>
-#include <folly/wangle/Future.h>
 
 namespace folly { namespace wangle {
 
@@ -98,4 +97,5 @@ private:
 
 }}
 
+#include <folly/wangle/Future.h>
 #include <folly/wangle/Promise-inl.h>

folly/wangle/Try.h

@@ -16,6 +16,10 @@
 
 #pragma once
 
+#include <type_traits>
+#include <exception>
+#include <algorithm>
+
 namespace folly { namespace wangle {
 
 template <class T>

folly/wangle/test/ClientCompile.cpp

+/*
+ * Copyright 2014 Facebook, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *   http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+// amazing what things can go wrong if you include things in an unexpected
+// order.
+#include <folly/wangle/Try.h>
+#include <folly/wangle/Promise.h>
+#include <folly/wangle/Future.h>
+int main() { return 0; }