all: rename Tlv namespace to tlv for consistency

This commit renames the Tlv namespace defined in
tlv.hpp to tlv to be more consistent with tlv::security
and tlv::nfd. A Tlv namespace alias is provided for
backwards compatibility.

Change-Id: I21278d6a077afe7776802c3296997d4c1c44c6c6
diff --git a/src/security/digest-sha256.cpp b/src/security/digest-sha256.cpp
index 6d450a2..3af551a 100644
--- a/src/security/digest-sha256.cpp
+++ b/src/security/digest-sha256.cpp
@@ -24,14 +24,14 @@
 namespace ndn {
 
 DigestSha256::DigestSha256()
-  : Signature(SignatureInfo(Tlv::DigestSha256))
+  : Signature(SignatureInfo(tlv::DigestSha256))
 {
 }
 
 DigestSha256::DigestSha256(const Signature& signature)
   : Signature(signature)
 {
-  if (getType() != Tlv::DigestSha256)
+  if (getType() != tlv::DigestSha256)
     throw Error("Incorrect signature type");
 }