Implement ft_col_count() along with corresponding unit tests

This pairs with the existing ft_row_count() using the same column
counting method used elsewhere in the code (i.e., iterate over rows
and find the biggest).

Added unit tests for ft_row_count(), ft_col_count(), ft_is_empty(),
and use a myriad of insertion methods to increase coverage.

src/fort.hpp

@@ -1172,6 +1172,18 @@ public:
         return ft_row_count(table_);
     }
 
+    /**
+     * Get number of columns in the table.
+     *
+     * @return
+     *   Number of columns in the table.
+     */
+    std::size_t
+    col_count() const noexcept
+    {
+        return ft_col_count(table_);
+    }
+
     /**
      * Get current cell.
      *