Allow an optional reason field to lint attributes

[brson]

Possibly like #[allow(types, reason = "whatever"). Then add another lint check that any allow attributes also have a reason.

[graydon]

+1 this should satisfy the most pedantic of pedants

[emberian]

Nominating for production ready.

[pnkfelix]

not accepted for milestone.

[emberian]

Assigning to myself; will work on today.

[catamorphism]

Heh, didn't realize this existed. Since #9025 was closed, this should be closed too.