Introduce with_*device functionality

The upcoming nitrokey 0.4 release changes the way a device handle can be
acquired, requiring a manager instance for doing so in an attempt to
prevent users from opening multiple sessions (which is not something
that libnitrokey supports).
A straight integration of the reworked API surface into our program
would severely complicate the architecture because of the additional
requirement of keeping a manager object around while a device is being
used.
To make the program more amenable to those changes in nitrokey, this
patch reworks the way we interact with a device handle: instead of
passing the device object around we pass in the functionality making use
of it in the form of a function. In more concrete terms, instead of
retrieving a device handle via get_device() we now have a with_device()
function that takes care of opening the device and then passing it to a
user-provided function.