Add reference count model to rasqal_bindings

(rasqal_new_bindings_from_bindings): Added
(rasqal_algebra_bindings_to_algebra): Take a reference to bindings
Bind variables for VALUES graph patterns

(rasqal_query_values_build_variables_use_map_binds): Added

(rasqal_query_graph_pattern_build_variables_use_map_binds): Call
above for VALUES graph patterns
(rasqal_algebra_values_algebra_node_to_rowsource): Add a bindings ref.
(rasqal_algebra_query_to_algebra): Execute top level select VALUES