euclidean domains PR

Stream: general

[Scott Morrison]

I think https://github.com/leanprover/mathlib/pull/78 is at this point waiting either to be merged or for further comments.

[Scott Morrison]

I hope eventually it can be merged, so I can tick off the last box for Louis' summer project.

[Mario Carneiro]

I merged Louis' PR. There are some minor things that I notice, but it's best to just let that stuff evolve inside mathlib, it's gone through the ringer already and it came out nicely