feat: improve efficiency of the decryptor

[aykevl]

The decryptor was using up a lot of time. For a ~10min song it took about twice the time that it took to run the Vorbis decoder and this time would only go up considering the exponential nature of the loop.

But actually, all it did was count up a number. This can be done using a simple addition instead of a complicated loop. Hence this patch changes the loop to a simple addition.

It is 32-bits at the moment, which means it would overflow with a stream that's 64GB 48GB big. This is entirely fixable if needed, but 48GB seems like an unlikely large stream. Updated to be 64 bits to make me feel better, this is extremely unlikely to ever overflow.

This replaces #120.

[aykevl]

Updated:

• Using binary.BigEndian instead of manually extracting/writing the individual bytes.
• Using a 64-bit counter instead of a 32-bit counter. This won't overflow with any reasonable stream size (it will only overflow after decrypting a stream that's more than 20 exabytes long).
• Improved the comment to explain why this is safe to do.

[devgianlu]

Love it!