Provide a public method to get StringRef's underlying pointer

This allows reducing the amount of friends needed for its interface
and some extra tricks later.

The bad part is that the pointer can become invalidated via
calls to other StringRef's public methods, but c'est la vie.

include/internal/catch_stringref.cpp

@@ -43,7 +43,7 @@ namespace Catch {
            const_cast<StringRef*>( this )->takeOwnership();
         return m_start;
     }
-    auto StringRef::data() const noexcept -> char const* {
+    auto StringRef::currentData() const noexcept -> char const* {
         return m_start;
     }
 
@@ -112,11 +112,11 @@ namespace Catch {
     }
 
     auto operator << ( std::ostream& os, StringRef const& str ) -> std::ostream& {
-        return os.write(str.m_start, str.m_size);
+        return os.write(str.currentData(), str.size());
     }
 
     auto operator+=( std::string& lhs, StringRef const& rhs ) -> std::string& {
-        lhs.append(rhs.m_start, rhs.m_size);
+        lhs.append(rhs.currentData(), rhs.size());
         return lhs;
     }