review? @brson

…le, r=phansch

Refactor lints in methods module

This PR refactors methods lints other than the lints I refactored in rust-lang/rust-clippy#6826 and moves some functions to methods/utils.rs.
Basically, I follow the instruction described in rust-lang#6680.

**For ease of review, I refactored step by step, keeping each commit small.**

closes rust-lang/rust-clippy#6886
cc: `@phansch,` `@flip1995,` `@Y-Nak`

changelog: Move lints in methods module to their own modules and some function to methods/utils.rs.

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.~