make tidy should require an issue number in FIXMEs

[nrc]

We used to do this and it was annoying, but it is more annoying not to have it. Let's reinstate it!

[nikomatsakis]

cc me

[Gankra]

There's a few FIXME's in libcollections that amount to "Future optimization/design work" that don't super make sense to have issues logged against them. I guess we could have a misc "make stuff better" bug.

[nrc]

We discussed this at the weekly meeting, and whilst we think we could do something better, requiring issue numbers when we haven't would be too much work and mean coming up with a lot of pointless issues. Reviewers should encourage issue numbers in FIXMEs, but we won't change make tidy for now.

[pnkfelix]

cc #3303 #11815