name: Add support for ImplicitSha256DigestComponent

This support includes the following new API functions:

- name::Component::isImplicitSha256Digest()
- name::Component::fromImplicitSha256Digest(...)
- Name::appendImplicitSha256Digest(...)
- Updated toUri()/fromUri(...) to support "sha256digest=..." URI
  representation of the ImplicitSha256Digest component

Change-Id: I756c4b94196cf031da98b5689bd60630533dfeb3
Refs: #1640, #2088
diff --git a/src/encoding/tlv.hpp b/src/encoding/tlv.hpp
index bb0f1c5..284ca26 100644
--- a/src/encoding/tlv.hpp
+++ b/src/encoding/tlv.hpp
@@ -61,6 +61,7 @@
   Interest      = 5,
   Data          = 6,
   Name          = 7,
+  ImplicitSha256DigestComponent = 1,
   NameComponent = 8,
   Selectors     = 9,
   Nonce         = 10,