Add -Wmissing-declarations to the SelfTest project

This required some clean-up in our test files

projects/SelfTest/UsageTests/ToStringWhich.tests.cpp

@@ -16,6 +16,16 @@ std::string fallbackStringifier(T const&) {
 #include "catch.hpp"
 
 
+
+#if defined(__GNUC__)
+// This has to be left enabled until end of the TU, because the GCC
+// frontend reports operator<<(std::ostream& os, const has_maker_and_operator&)
+// as unused anyway
+#    pragma GCC diagnostic ignored "-Wunused-function"
+#endif
+
+namespace {
+
 struct has_operator { };
 struct has_maker {};
 struct has_maker_and_operator {};
@@ -38,6 +48,8 @@ StreamT& operator<<(StreamT& os, const has_template_operator&) {
     return os;
 }
 
+} // end anonymous namespace
+
 namespace Catch {
     template<>
     struct StringMaker<has_maker> {
@@ -100,6 +112,8 @@ TEST_CASE( "stringify( vectors<has_maker_and_operator> )", "[toString]" ) {
     REQUIRE( ::Catch::Detail::stringify( v ) == "{ StringMaker<has_maker_and_operator> }" );
 }
 
+namespace {
+
 // Range-based conversion should only be used if other possibilities fail
 struct int_iterator {
     using iterator_category = std::input_iterator_tag;
@@ -139,6 +153,8 @@ struct stringmaker_range {
     int_iterator end() const { return {}; }
 };
 
+} // end anonymous namespace
+
 namespace Catch {
 template <>
 struct StringMaker<stringmaker_range> {
@@ -148,6 +164,8 @@ struct StringMaker<stringmaker_range> {
 };
 }
 
+namespace {
+
 struct just_range {
     int_iterator begin() const { return int_iterator{ 1 }; }
     int_iterator end() const { return {}; }
@@ -158,6 +176,8 @@ struct disabled_range {
     int_iterator end() const { return {}; }
 };
 
+} // end anonymous namespace
+
 namespace Catch {
 template <>
 struct is_range<disabled_range> {