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/SecretKey.cpp

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