encoding: Improving structure and documentation of block helpers

This commit add several new helpers to simplify operations with
std::string:

- prependStringBlock
- makeStringBlock
- readString

The following functions are deprecated and their replacements:

- nonNegativeIntegerBlock (use makeNonNegativeIntegerBlock)
- prependBooleanBlock (use prependEmptyBlock)
- booleanBlock (use makeEmptyBlock)
- dataBlock (use makeBinaryBlock)
- nestedBlock (use makeNestedBlock)

Change-Id: Ic595ae64fc9c80c2c04e5fde1d8812e8b9debd60
Refs: #2951
diff --git a/tests/unit-tests/make-interest-data.hpp b/tests/unit-tests/make-interest-data.hpp
index 23fcc22..ca3bff5 100644
--- a/tests/unit-tests/make-interest-data.hpp
+++ b/tests/unit-tests/make-interest-data.hpp
@@ -44,8 +44,7 @@
 signData(const shared_ptr<Data>& data)
 {
   ndn::SignatureSha256WithRsa fakeSignature;
-  fakeSignature.setValue(ndn::dataBlock(tlv::SignatureValue,
-                                        reinterpret_cast<const uint8_t*>(0), 0));
+  fakeSignature.setValue(makeEmptyBlock(tlv::SignatureValue));
   data->setSignature(fakeSignature);
   data->wireEncode();