base: ensure Element::Error inherits from tlv::Error

Adding static asserts to ensure (Name|name::Component|Interest|
Selectors|Exclude|KeyLocator|Data|MetaInfo|Signature|SignatureInfo)
::Error are subclasses of tlv::Error.

This commit also adds WireEncodable and WireDecodable concept checks
to Name,name::Component,Interest,Selectors,Exclude,KeyLocator,Data,
MetaInfo,SignatureInfo types.

This commit also moves definition of Selectors method into .cpp.

refs #1983

Change-Id: I15220b93437d4a624ae09df66defc91ceac2386c
diff --git a/src/key-locator.cpp b/src/key-locator.cpp
index ba17771..ddc0029 100644
--- a/src/key-locator.cpp
+++ b/src/key-locator.cpp
@@ -21,9 +21,16 @@
 
 #include "key-locator.hpp"
 #include "encoding/block-helpers.hpp"
+#include "util/concepts.hpp"
 
 namespace ndn {
 
+BOOST_CONCEPT_ASSERT((boost::EqualityComparable<KeyLocator>));
+BOOST_CONCEPT_ASSERT((WireEncodable<KeyLocator>));
+BOOST_CONCEPT_ASSERT((WireDecodable<KeyLocator>));
+static_assert(std::is_base_of<tlv::Error, KeyLocator::Error>::value,
+              "KeyLocator::Error must inherit from tlv::Error");
+
 KeyLocator::KeyLocator()
   : m_type(KeyLocator_None)
 {