new lint: Unexpected double indirection

In Rust it's relatively easy to accidentally end up with a &&T type when expecting to use &T.

The problem is that thanks to auto dereferencing &&T most of the time works just as well as &T, so the difference may go unnoticed for a while. But later when the code is changed, it may get weird type errors in unexpected places somewhere later in the code:

let s = "foo";
let s = &s[1..2]; // ok
let s = &s[1..2].trim(); // &&str suddenly
let l = s.len(); // but seems to work…

if s == "oo" // except, "can't compare `&str` with `str`"!?

for s in ["a", "b"].iter() { // gives &&str

   let s: String = s.to_owned(); // error: makes "owned" &str
}

So I'm hoping Clippy could find patterns which typically cause unexpected creation of doubly-indirect types, and warn about them before they become a hard-to-diagnose problem later.

Start from the two Rust examples in the issue and identify which expressions create unexpected &&T values through borrowing or iteration. Define the lint's supported patterns and warnings, then verify that it catches the demonstrated cases without flagging intentional double indirection.