Warn on unused/unnecessary trait bounds

[gsingh93]

I sometimes overspecify the trait bounds on an impl by accident. In many cases, some of the bounds aren't actually needed. It would be nice to have a lint I could run to warn me whenever I've done this so I could clean it up.

There are cases when you want to overspecify the trait boundaries so you can add things to them without breaking backwards compatibility, but in these cases you'd just ignore the lint or turn it off.

[llogiq]

We should tread lightly here. For example it's hard to know if e.g. marker traits are really needed. E.g. do we really need Ord or is PartialOrd sufficient?