Problem: message_t to string is hard

Solution: add to_string() function.

tests/message.cpp

@@ -151,6 +151,18 @@ TEST_CASE("message equality non equal lhs empty", "[message]")
     CHECK(msg_a != msg_b);
 }
 
+TEST_CASE("message to string", "[message]")
+{
+    const zmq::message_t a;
+    const zmq::message_t b("Foo", 3);
+    CHECK(a.to_string() == "");
+    CHECK(b.to_string() == "Foo");
+#ifdef ZMQ_CPP17
+    CHECK(a.to_string_view() == "");
+    CHECK(b.to_string_view() == "Foo");
+#endif
+}
+
 #if defined(ZMQ_BUILD_DRAFT_API) && ZMQ_VERSION >= ZMQ_MAKE_VERSION(4, 2, 0)
 TEST_CASE("message routing id persists", "[message]")
 {

zmq.hpp

@@ -552,9 +552,25 @@ class message_t
     }
 #endif
 
-    /** Dump content to string. Ascii chars are readable, the rest is printed as hex.
-         *  Probably ridiculously slow.
-         */
+    // interpret message content as a string
+    std::string to_string() const
+    {
+        return std::string(static_cast<const char*>(data()), size());
+    }
+#ifdef ZMQ_CPP17
+    // interpret message content as a string
+    std::string_view to_string_view() const noexcept
+    {
+        return std::string_view(static_cast<const char*>(data()), size());
+    }
+#endif
+
+    /** Dump content to string for debugging.
+    *   Ascii chars are readable, the rest is printed as hex.
+    *   Probably ridiculously slow.
+    *   Use to_string() or to_string_view() for
+    *   interpreting the message as a string.
+    */
     std::string str() const
     {
         // Partly mutuated from the same method in zmq::multipart_t