Removed inefficient setdiff calls

[dlcole3]

This PR addresses #124. Adding link constraints to a graph with lots of variables was inefficient because of the setdiff call that was called by the _add_backend_variables function (called by MOI.add_constraint). I replaced the setdiff with a for loop that tests whether the variable is in the keys of backend.element_to_graph_map.var_map rather than building a new vector of these variables first. The code snippet in #124 now works much quicker and the time for running each iteration of the loop in that issue no longer increases after each iteration.