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/base_uint.h

@@ -510,7 +510,7 @@ inline const base_uint<Bits, Tag> operator+ (
 template <std::size_t Bits, class Tag>
 inline std::string to_string (base_uint<Bits, Tag> const& a)
 {
-    return strHex (a.begin (), a.size ());
+    return strHex (a.cbegin (), a.cend ());
 }
 
 // Function templates that return a base_uint given text in hexadecimal.