Remove clone call

[PeterMonsson]

Remove clone call that is not needed.

(Background: I have run svlint on a large code base. The total run time was 14 minutes. I tried profiling the code and noticed that the top 3 functions were memory allocations. I have not had the time to find out exactly where those memory allocations are coming from. This is my best shot at improving performance with the limited time that I have had.

For comparison the verible-verilog-lint runs in less than a second)

[PeterMonsson]

Unfortunately, this is not where the allocations are happening. A profile with cargo flamegraph indicates that the performance is dominated by clone() and insert() in sv-parser's preprocess_str:

• 2 inserts
  defines.insert(k.to_string(), Some(define));
  defines.insert(k.clone(), (*v).clone());

• 5 clones
  defines.insert(k.clone(), (*v).clone());
  match n.clone() {
  match n.clone() {

                let define = Define {
                    identifier: id.clone(),
                    arguments: define_args,
                    text: define_text,
                };