Warn on circular code

What it does

When code goes in a circular loop (fn1 calls fn 2, which then calls fn1), Clippy will emit a warning about this behavior, so the programmer won't have to wonder why their program apparently hangs.

In debug mode, it will crash with the following:

thread 'main' has overflowed its stack
fatal runtime error: stack overflow

This would be something along the lines of unconditional_recursion in rustc, unsure if these checks need to be in two separate places.

See https://github.com/rust-lang/rust-clippy/issues/428 for a previous issue.

Advantage

Circular Rust code now won't compile without the programmer knowing it's circular, but can still do so if desired.

Drawbacks

None

Example

fn output1() {
    output2();
}

fn output2() {
    output1();
}

fn main() {
    output1();
}

No files or tests are named. Start with the mutually recursive example, compare the proposed behavior with rustc's unconditional_recursion, and read the referenced previous issue; done means circular calls produce a warning while preserving the stated ability to compile intentionally.