Internal utility for visiting all returns

[GnomedDev]

We should have a ReturnVisitor built on the intravisit::Visitor infra to visit all explicit ExprKind::Return and implicit (last expression in a block) returns.

This would significantly help with some lint ideas, and like #305 (came up in #13395) and #12290 to name a couple.

[GnomedDev]

@rustbot claim