Move-enable various parts of TextFlow

This removes about 200 pointless copies from printing the help
message (the original motivation for the change), and also nicely
improves performance of the various reporters that depend on
TextFlow.

src/catch2/internal/catch_textflow.cpp

@@ -233,23 +233,36 @@ namespace Catch {
             return os;
         }
 
-        Columns Column::operator+( Column const& other ) {
+        Columns operator+(Column const& lhs, Column const& rhs) {
             Columns cols;
-            cols += *this;
-            cols += other;
+            cols += lhs;
+            cols += rhs;
+            return cols;
+        }
+        Columns operator+(Column&& lhs, Column&& rhs) {
+            Columns cols;
+            cols += CATCH_MOVE( lhs );
+            cols += CATCH_MOVE( rhs );
             return cols;
         }
 
-        Columns& Columns::operator+=( Column const& col ) {
-            m_columns.push_back( col );
-            return *this;
+        Columns& operator+=(Columns& lhs, Column const& rhs) {
+            lhs.m_columns.push_back( rhs );
+            return lhs;
         }
-
-        Columns Columns::operator+( Column const& col ) {
-            Columns combined = *this;
-            combined += col;
+        Columns& operator+=(Columns& lhs, Column&& rhs) {
+            lhs.m_columns.push_back( CATCH_MOVE(rhs) );
+            return lhs;
+        }
+        Columns operator+( Columns const& lhs, Column const& rhs ) {
+            auto combined( lhs );
+            combined += rhs;
             return combined;
         }
+        Columns operator+( Columns&& lhs, Column&& rhs ) {
+            lhs += CATCH_MOVE( rhs );
+            return CATCH_MOVE( lhs );
+        }
 
     } // namespace TextFlow
 } // namespace Catch