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/util/test-in-memory-storage-common.cpp b/tests/unit-tests/util/test-in-memory-storage-common.cpp
index 5fe9805..f2c1b9f 100644
--- a/tests/unit-tests/util/test-in-memory-storage-common.cpp
+++ b/tests/unit-tests/util/test-in-memory-storage-common.cpp
@@ -510,8 +510,7 @@
   const ndn::KeyLocator locator(keyName);
 
   ndn::SignatureSha256WithRsa fakeSignature;
-  fakeSignature.setValue(ndn::dataBlock(tlv::SignatureValue,
-                                        reinterpret_cast<const uint8_t*>(0), 0));
+  fakeSignature.setValue(makeEmptyBlock(tlv::SignatureValue));
 
   fakeSignature.setKeyLocator(locator);
   data2->setSignature(fakeSignature);