add `let_underscore_lock` lint

closes rust-lang#1574
changelog: add `let_underscore_lock` lint

I am not entirely sure about my docs/messages wording here, improvements are welcome

Add `let_underscore_drop`

This line generalizes `let_underscore_lock` (rust-lang#5101) to warn about any initializer expression that implements `Drop`.

So, for example, the following would generate a warning:
```rust
struct Droppable;
impl Drop for Droppable {
    fn drop(&mut self) {}
}
let _ = Droppable;
```

I tried to preserve the original `let_underscore_lock` functionality in the sense that the warning generated for
```rust
let _ = mutex.lock();
```
should be unchanged.

*Please keep the line below*
changelog: Add lint [`let_underscore_drop`]