New lint: let statement with no effect

[CAD97]

As featured in this Twitter poll.

A statement of the form let _ = <place> has no effect (modulo Deref coersion) as the place is not moved from. This should instead be omited (if all Deref coersion is pure) or otherwise written to be more clear on intent (explicitly calling side-effectful Deref).

50% of people had the wrong intuition of behavior here, so this definitely would help avoid potentially misleading code.

[mati865]

let _ = expr is also used to mute must_use lint.

[CAD97]

let _ = $expr is perfectly fine for an expr that has an effect (e.g. calls a method). It's specifically "path expressions" that are problematic, as they lead to a no-op (modulo deref coersions) statement.