Refactor string handling in util

The util module provides helper methods to deal with the C strings
returned by libnitrokey.  The current implementation has to problems:
- It causes unnecessary allocations if we only want to look at the
  string, for example in get_serial_number.
- If the conversion from a CStr to a String fails, the string pointer
  is not freed.

Therefore this patch introduces the run_with_str function that executes
a function with the string returned by libnitrokey and then makes sure
that the pointer is freed correctly.

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