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/STPathSet.h

@@ -120,7 +120,7 @@ public:
     STPathElement(STPathElement const&) = default;
     STPathElement& operator=(STPathElement const&) = default;
 
-    int
+    auto
     getNodeType () const
     {
         return mType;