fix/feat: Change the existing Add<...> impls

[ParkMyCar]

Implementing Add<CompactString> for String breaks String + &String, as long as compact_str is used somewhere in your current crate. Specifically this code won't compile:

use compact_str;

fn main() {
  println!("{}", "Hello".to_string() + &(" World".to_string()));
}

This isn't good! This has to do with Rust's current trait resolver and is a known issue rust-lang/rust#77143

To fix this, we remove impl Add<CompactString> for String. You can still add CompactString to String, you just need to take a reference to it, then the impl Add<&str> for String kicks in.

We also remove a lot of impl Add<...> for CompactString, leaving just impl Add<&str> for CompactString, which allows us to add more string types than we currently can, e.g. &Box<str> and &Cow<str>. This is because the compiler auto derefs these types to &str and uses that impl

[ParkMyCar]

Note: these Add<...> impls were first available on v0.4.0. This fix has been back ported to:

• v0.4.0 -> v0.4.1
• v0.5.0 -> v0.5.1

Technically this was a breaking change because you can no longer add a String to a CompactString, but to fix it all you need to do is take a reference, e.g. make it &String. IMO this small change is worth it to prevent the bug, where importing compact_str can break other String + operations, even if they don't use CompactString at all themselves.