lib: use buffer-xor over inline

[dcousens]

I've written this function enough times :), why not? Eh @jprichardson haha

edit: Maybe wait out on browserify/buffer-xor#1?

[dcousens]

It is interesting, because in other languages with a less verbose syntax, it might just be as simple as:

D-lang

int[3] c = a[] ^ b[]

Or say in Python w/ numpy:

c = numpy.bitwise_xor(a, b)

Or if there wasn't that overhead for functional idioms, and the lack of decent binary arrays in JS, using ES6:

c = a.map((x, i) => a[i] ^ b[i])

But, alas. We don't have these things, and this stuff can be finicky. So, what do?

It is 10 lines of code, that were previously untested in isolation, they are now tested and consistent.

[coveralls]

Coverage remained the same at 100.0% when pulling 4852427 on bufxor into 9c4af83 on master.

[dcousens]

@jprichardson thoughts?

[jprichardson]

Yeah, I'm fine with it.