Feature request: detect and suggest discards in tuple deconstruction

Code along these lines:

```csharp
var (foo, bar, quux) = MethodThatReturnsATwoTuple();

foo.DoSomething();
// bar is never used and could be discarded
quux.AnotherThing();
```

could be rewritten as:

```csharp
var (foo, _, quux) = MethodThatReturnsATwoTuple();

foo.DoSomething();
quux.AnotherThing();
```