Allow key to be a TypedArray

[gabegorelick]

Earlier work allowed passphrase to be a TypedArray. This extends that to allow for key to be too.

See #5

[chrisveness]

If Scrypt.key() returns a Buffer, which it is up to the user to convert to a string, it doesn't seem right to assume that if a string is passed to Scrypt.verify() that it will be base64 – it could be hex, for example (though I cannot see why a user would prefer to store a key as hex in preference to base64!).

Perhaps it would make most sense for Scrypt.verify() to accept the key as a Buffer only, and the user would be required to convert the string to a buffer, as in

const key = await Scrypt.kdf('my secret password', { logN: 15 }).toString('base64');
const ok = await Scrypt.verify(Buffer.from(key, 'base64'), 'my secret password');

[demurgos]

@chrisveness It makes sense, by accepting only buffers you reduce the assumptions made by the lib.

[gabegorelick]

I agree with all the comments here. I wrote this PR to be backwards compatible. But assuming that's not something we care about (because it's being broken to return Buffers instead of strings), then I think only accepting Buffers (or TypedArrays) makes sense.

We could merge this and then remove String support though. But up to you how you want to manage the changes.

[chrisveness]

Thanks for the PR, but I decided it would be easier to base the new Buffer version off the current version.