make the Salt fields public

[lucascompython]

This way one could store the bytes of the salt, that are most likely smaller than the b64 representation, and you skip the cost of encoding and decoding.

[tarcieri]

I'd really prefer to keep the implementation details of the type private.

[lucascompython]

Can I ask why?

[tarcieri]

It means the internal representation can't be changed, for example there's a lot of duplication between Salt and StringBuf and they could perhaps use a common internal type.

It would also break the derived Eq/PartialEq implementations since one of the buffers may contain junk at the end and the other one is zeros, but the bytes that are actually within the specified length of the salt are the same and they should compare equal.