Provide per package semver bump suggestions

[drahnr]

This might be a very difficult problem, there is some rust nightly support.

Conceptual idea:

• track re-exports that change
• track changes between type definitions under same paths
• track changed and incompatible trait bound changes
• changing pub trait default impl

this information can the be used to bump server at the leafs in the project and trickle it up accordingly.

Note that this might be very well out of scope and be better situated in another tool and/or rustc, so feel free to close

[gnunicorn]

#25 and #31 where both trying to get there. it is really quite a complex task...