Simplify strHex:

Problem:
- There are several specific overloads with some custom code that can be
  easily replaced using Boost.Hex.

Solution:
- Introduce `strHex(itr, itr)` to return a string given a begin and end
  iterator.
- Remove `strHex(itr, size)` in favor of the `strHex(T)` where T is
  something that has a `begin()` member function. This allows us to
  remove the strHex overloads for `std::string`, Blob, and Slice.

src/ripple/protocol/impl/PublicKey.cpp

@@ -31,7 +31,7 @@ namespace ripple {
 std::ostream&
 operator<<(std::ostream& os, PublicKey const& pk)
 {
-    os << strHex(pk.data(), pk.size());
+    os << strHex(pk);
     return os;
 }