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.

PR-URL: https://github.com/nodejs/node/pull/5441


Reviewed-By: Brian White <mscdex@mscdex.net>
Reviewed-By: Evan Lucas <evanlucas@me.com>