field_sensitivityt::apply does not take a reference

[romainbrenguier]

We make apply take the expression by copy and return a transformed one.
This makes it more intuitive to use and avoid problems of the kind:

      index_exprt i;
      apply(ns, s, i, true);
      f_of_index_expr(i);

where in the call to f_of_index_expr, i is of type index_exprt but may not have
ID_index thus breaking an invariant which we should intuitively have.

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