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/basics/impl/strHex.cpp

@@ -17,9 +17,9 @@
 */
 //==============================================================================
 
-#include <ripple/basics/Slice.h>
 #include <ripple/basics/strHex.h>
 #include <algorithm>
+#include <string>
 
 namespace ripple {
 
@@ -51,10 +51,4 @@ int charUnHex (unsigned char c)
     return xtab[c];
 }
 
-std::string
-strHex(Slice const& slice)
-{
-    return strHex(slice.data(), slice.size());
-}
-
 }