Only parse 64 bit int

[nmhancock]

I ran into an issue while implementing balloon hashing in javascript using this as a reference.

The issue resolved down to getting different values for other, and that was because the python implementation generates a bigint from the entire buffer, rather than reading a 64 bit integer as in the C reference implementation.

Note: This commit breaks all the unit tests, so if those test vectors are known good then the code below is a mistake. But if they're not this may be an issue.

[daxpedda]

For various reasons, see RustCrypto/password-hashes#232, we are not following the C reference implementation.

There are already a bunch of libraries out there that have used this implementation with the test vectors so breaking that isn't really an option unless it's for a good reason.

I don't remember all the details, but looking at the paper it's not exactly clear how this should be done. I'm trying to retrace the steps I originally took in my mind and it seems to me that how it's currently done discards the least amount of data, which is probably why I decided to go that route.

If you are interested, you could adjust the reference implementation to use this algorithm, which was approved by the author: henrycg/balloon#1 (comment).