Use StringRef in pluralise implementation

This means that it can no longer be safely made ahead of time,
but nothing in our existing code used it like that. Normally it
is constructed and used in the same expression, which is now
more efficient.

src/catch2/internal/catch_string_manip.cpp

@@ -94,11 +94,6 @@ namespace Catch {
         return subStrings;
     }
 
-    pluralise::pluralise( std::size_t count, std::string const& label )
-    :   m_count( count ),
-        m_label( label )
-    {}
-
     std::ostream& operator << ( std::ostream& os, pluralise const& pluraliser ) {
         os << pluraliser.m_count << ' ' << pluraliser.m_label;
         if( pluraliser.m_count != 1 )

src/catch2/internal/catch_string_manip.hpp

@@ -32,13 +32,26 @@ namespace Catch {
     std::vector<StringRef> splitStringRef( StringRef str, char delimiter );
     bool replaceInPlace( std::string& str, std::string const& replaceThis, std::string const& withThis );
 
+    /**
+     * Helper for streaming a "count [maybe-plural-of-label]" human-friendly string
+     *
+     * Usage example:
+     * ```cpp
+     * std::cout << "Found " << pluralise(count, "error") << '\n';
+     * ```
+     *
+     * **Important:** The provided string must outlive the instance
+     */
     struct pluralise {
-        pluralise( std::size_t count, std::string const& label );
+        pluralise(std::size_t count, StringRef label):
+            m_count(count),
+            m_label(label)
+        {}
 
         friend std::ostream& operator << ( std::ostream& os, pluralise const& pluraliser );
 
         std::size_t m_count;
-        std::string m_label;
+        StringRef m_label;
     };
 }