removed before releasing the wrapper code.  Compiler will detect if
any of them is forgotten to be cleaned up.

(there doesn't seem to be need for updating the python doc)

as suggested in review.  hid the concrete derived class in the implementation.
adjusted tests accordingly.

we should either/both consider revisiting the interface so we avoid the
issue as a result or require higher version of boost.  but for now this
should be okay.

So we can distinguish the errors.

fixing bugs like case-sensitivity or corner cases for empty salts.
provided more detailed tests and documentation.

not directly related but would be helpful for the purpose of the branch.