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;
 }
 

src/ripple/protocol/impl/STBlob.cpp

@@ -31,7 +31,7 @@ STBlob::STBlob (SerialIter& st, SField const& name)
 std::string
 STBlob::getText () const
 {
-    return strHex (value_.data (), value_.size ());
+    return strHex (value_);
 }
 
 bool

src/ripple/protocol/impl/STPathSet.cpp

@@ -156,7 +156,7 @@ STPath::getJson (int) const
     for (auto it: mPath)
     {
         Json::Value elem (Json::objectValue);
-        int         iType   = it.getNodeType ();
+        auto const iType   = it.getNodeType ();
 
         elem[jss::type]      = iType;
         elem[jss::type_hex]  = strHex (iType);

src/ripple/protocol/impl/SecretKey.cpp

@@ -51,7 +51,7 @@ SecretKey::SecretKey (Slice const& slice)
 std::string
 SecretKey::to_string() const
 {
-    return strHex(data(), size());
+    return strHex(*this);
 }
 
 //------------------------------------------------------------------------------

src/ripple/protocol/impl/UintTypes.cpp

@@ -61,7 +61,7 @@ std::string to_string(Currency const& currency)
         }
     }
 
-    return strHex (currency.begin (), currency.size ());
+    return strHex (currency);
 }
 
 bool to_currency(Currency& currency, std::string const& code)