Lint to disallow unsigned subtraction

[ppershing]

Lint explanation

Unsigned subtraction is likely to introduce underflow bugs. These bugs won't be caught in release mode.

Example code

Lint should disallow following code

let len: usize = vec.len();
let max_len: usize = 1000;
let available_space = max_len - len; // likely to underflow

while allowing both

let space = max_len.wrapping/saturating/checked_sub(len)

and

let len: isize = vec.len() as i32;
let max_len: isize = 1000;
let space = max_len - len;

[xFrednet]

Thank you for the issue. This lint should already be possible, I want to create some example lints and this one might be a good and simple example. I'll implement it after the next release at the start of October.

Btw, you might want to take a look at clippy::arithmetic_side_effects which checks for all arithmetic operators that can cause a panic