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/test/rpc/AccountSet_test.cpp

@@ -212,7 +212,8 @@ public:
         env(jt);
         BEAST_EXPECT(! env.le(alice)->isFieldPresent(sfMessageKey));
 
-        jt[sfMessageKey.fieldName] = strHex("NOT_REALLY_A_PUBKEY");
+        using namespace std::string_literals;
+        jt[sfMessageKey.fieldName] = strHex("NOT_REALLY_A_PUBKEY"s);
         env(jt, ter(telBAD_PUBLIC_KEY));
     }