Implement comparison of two deepdep objects

[ErdaradunGaztea]

Description of a problem related to your idea
I'd like to compare differences between dependencies used in different versions of a package; e.g. differences between deepdep v0.2.0 and deepdep v0.4.1.

Solution you'd like
I'd want a function called diffdep() that takes two deepdep objects and returns a list with the following fields:

• added: for packages that weren't dependencies before, but now are
• removed: for packages that aren't dependencies anymore
• upgraded & downgraded: for changes of minimum required version of a package
• promoted & demoted or simply changed: for moving packages between different dependency types, e.g. from Suggests to Imports