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 )