fix(data/encodable): make decidable_eq_of_encodable priority 0

[kbuzzard]

Should I also add some unit test or something?

[johoelzl]

I don't think a unit test is necessary, we generally don't have them in mathlib. But I would like to see a comment why priority 0 it is required. In a couple of weeks nobody will remember its reason.

[kbuzzard]

Sorry this took forever. [I must be honest -- when I realised real mathlib had more commits than my fork I used this as an excuse to learn about git in general (clone, fork, merge, rebase, managing master + origin/master + remote/master)]. I added a comment.

[added later] Hmm. So did I not have to merge my suggested edit with mathlib in its current state?

[johoelzl]

This is now in the repo.

Sorry for the delay!