(rasqal_new_results_compare): Allocate defined_in_map 2x larger
Compute sequence of variable names for row on demand

(rasqal_row_get_names): Added internal method
(rasqal_query_results_get_bindings): Use rasqal_row_get_names()
(rasqal_variables_table_get_names): Remove and variable_names field
Make rowsource usage counted

(rasqal_new_rowsource_from_rowsource): Added
(rasqal_new_rowsource_from_handler): set usage earlier
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