core: drop std::function and ndn::Exclude from common.hpp, add ndn::optional

Change-Id: Ie3983d8b6f9929430efd8ada8d942e3f95755cd4
diff --git a/tests/daemon/fw/forwarder.t.cpp b/tests/daemon/fw/forwarder.t.cpp
index a5118d8..82a93ea 100644
--- a/tests/daemon/fw/forwarder.t.cpp
+++ b/tests/daemon/fw/forwarder.t.cpp
@@ -190,7 +190,7 @@
 
 protected:
   void
-  dispatchToStrategy(pit::Entry& pitEntry, function<void(fw::Strategy&)> trigger) override
+  dispatchToStrategy(pit::Entry&, std::function<void(fw::Strategy&)>) override
   {
     ++dispatchToStrategy_count;
   }