util: reverse buffers instead of strings

[pinheadmz]

This PR is just a cleanup from the recent buffers-instead-of-strings switchover.
Although these methods do not get called rapidly (they're mainly for JSON responses and API calls)
@BluSyn was able to observe a performance improvement:

~50% faster when compared over 1000000 iterations
difference is ~1300ms vs. ~600ms over 1m iterations

[tynes]

Good find!

[braydonf]

Is it worth it to include a small benchmark in ./bench?

[pinheadmz]

@braydonf Bench test added, results testing against master:

--> git checkout master
--> node bench/hexstring.js
hexstring: ops=100000, time=0.298963409, rate=334489.09462
--> git checkout hashstring
--> node bench/hexstring.js
hexstring: ops=100000, time=0.140646259, rate=711003.62506

[braydonf]

Looks like a 2 to 2 1/4 times faster, see about the same results.

[braydonf]

Was taking a look at when reverse() was introduced (I think since v4) at the Node.js documentation, and the method is apparently not documented, see https://nodejs.org/docs/latest-v10.x/api/buffer.html

[braydonf]

Okay, so it's not documented because Buffer extends Uint8Array and Uint8Array is documented.

[pinheadmz]

Ok great, thanks for checking. was just going to add, reverse() is working in node version 8.5 and above

[braydonf]

Yeah, it's available from v4 and above (and possible some of the iojs releases in-between).