interest+data: add string_view overloads for convenience

Also, explicitly declare the nullptr_t overloads as deleted, so that
users get a clear compilation error instead of a potentially confusing
"ambiguous overload" if they try to pass nullptr to these functions.

Change-Id: I399efcbd698a0d3f7e29219d21eb1a53a1d71070
diff --git a/tests/unit/security/key-chain.t.cpp b/tests/unit/security/key-chain.t.cpp
index d0b717f..c154851 100644
--- a/tests/unit/security/key-chain.t.cpp
+++ b/tests/unit/security/key-chain.t.cpp
@@ -711,7 +711,7 @@
   BOOST_CHECK_THROW(signerKeyChain.makeCertificate(request, signerParams), std::invalid_argument);
 
   // empty content
-  request.setContent(span<uint8_t>{});
+  request.setContent("");
   BOOST_CHECK_THROW(signerKeyChain.makeCertificate(request, signerParams), std::invalid_argument);
 }