add coq-stdpp as dependency, add abs_examples module

[palmskog]

Let's add Std++ as a dependency so we get access to the nice by finisher and other nice stuff. by is used to guarantee that a goal is properly discharged and applies some heuristics like auto.

This should be easy for you to rebase over, right @Aqissiaq?

I will do a followup PR later where I add arithmetic and boolean expressions and some more examples of typing proofs.