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/encoding/block-helpers.hpp b/src/encoding/block-helpers.hpp
index 8163d0c..0a58aa0 100644
--- a/src/encoding/block-helpers.hpp
+++ b/src/encoding/block-helpers.hpp
@@ -45,7 +45,7 @@
 readNonNegativeInteger(const Block& block)
 {
   Buffer::const_iterator begin = block.value_begin();
-  return Tlv::readNonNegativeInteger(block.value_size(), begin, block.value_end());
+  return tlv::readNonNegativeInteger(block.value_size(), begin, block.value_end());
 }
 
 inline Block
@@ -87,8 +87,8 @@
 //     ++dataSize;
 
 //   OBufferStream os;
-//   Tlv::writeVarNumber(os, type);
-//   Tlv::writeVarNumber(os, dataSize);
+//   tlv::writeVarNumber(os, type);
+//   tlv::writeVarNumber(os, dataSize);
 //   std::copy(first, last, std::ostream_iterator<uint8_t>(os));
 
 //   return Block(os.buf());