Add useless_conversion

[renzobanegass]

What it does

Checks for unnecesary uses of conversions.

Why is this bad?

Unnecessary conversions increase the complexity of the code, making it harder to read and maintain. They can also lead to unnecessary operations at runtime.

Example

// format!() returns a `String`
let s: String = format!("hello").into();

Instead do:

let s: String = format!("hello");