Use `write_str` instead of `write!` or `write_fmt` when format has no args

What it does

Despite amazing efforts by @m-ou-se, it seems the generated assembly code for write!(f, "something") is still far more complex (and likely slower) than f.write_str("something"). While eventually I do hope it will get solved, I think Clippy can already help users to improve their code performance by suggesting to switch to write_str where possible.

Advantage

• faster code
• smaller compiled code

Drawbacks

• in some cases may break consistency of only using write!
• some day may become irrelevant (hopefully soon, but I wont bet on it)

Example

use std::fmt;

pub struct Obj(String);

impl fmt::Display for Obj {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        write!(f, "AAAAAAAAAAAAAAAAAAAAAA")?;
        write!(f, "{}", self.0)?;
        let s: &str = &self.0;
        write!(f, "{s}")
    }
}

Could be written as:

    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        f.write_str("AAAAAAAAAAAAAAAAAAAAAA")?;
        f.write_str(&self.0)?;
        let s: &str = &self.0;
        f.write_str(s)
    }

Start by locating Clippy's handling of write! and write_fmt calls and review the linked Rust issue and Compiler Explorer example for the performance motivation. Add coverage for the examples shown, including literal and argument-free formatting, and verify that the lint suggests write_str only when no formatting arguments are needed.