benchmark: add benchmark for buf.compare()

There is a benchmark for the class method Buffer.compare() but not for the instance method buf.compare(). This adds that benchmark.

I used this to confirm a performance regression in an implementation I was considering. While the implementation was a bust, it does seem like the benchmark is worthwhile.

The benchmark is nearly identical to the existing Buffer.compare() benchmark except, of course, that it calls buf.compare() instead.