Add PartialEq, Eq, and Hash for Row and Col structs

[c123w]

I've found this useful when creating problems with a large number of variables or constraints, so that I can manage them and keep track of what they mean.

e.g. I have a foos: Vec<Foo> and I create a variable: Col with problem.add_integer_column() for each foo in foos. I'd like to be able to create a HashMap<Col, &Foo> so that I can keep track of which Foo corresponds to each Col.