Add trait to extend store for link consistency check
Add trait to extend store for link consistency check

This commit adds the error infrastructure as well as the trait for
extensing the `Store` type with a function to do a link consistency
check over _all_ entries in the store.

The functionality is not implemented yet.

mario-kr/imag,mario-kr/imag,matthiasbeyer/imag,matthiasbeyer/imag