Closes #6607

I went with `RawPtr` instead of `UnsafePtr` because not all of these operations are `unsafe`, so to me it makes more sense to refer to it as a "raw" (not wrapped/abstracted) pointer. If we decide on something else in #6608 it can be renamed again.

Destructure args in `methods`

changelog: none

This changes the main pattern in `methods` to match and destructure the method call args at the same time as the method name, and pass individual arg `Expr`s to the lint impls.

```rust
// before
["expect", ..] => expect::check(cx, expr, arg_lists[0]);
// after
("expect", [arg]) => expect::check(cx, expr, recv, arg);
```

This makes the code safer since there is no risk of out of bounds `args[n]` everywhere. There will be no more collecting `method_names`, `arg_lists`, `method_spans` as a separate step - everything comes out of the `match`es. Chained methods are parsed in a nested `match`. This makes the code more verbose in some ways, but IMO it is much easier to follow.

~Definitely should wait for rust-lang#6896. Just putting out the idea.~