Add a type for representing raw-agnostic format vars

[dtolnay]

The core::fmt format string syntax considers format_args!("...", r#keyword = "...") as equivalent to format_args!("...", keyword = "..."). That means manipulating named argument sets as Set<Ident> using Ident's Ord impl doesn't make sense, as that impl considers r#keyword != keyword.

Before this PR, the following would compile successfully because thiserror would see that fn was already present in the user-provided named arguments, so it would not try to insert a new named argument referring to the struct field:

#[derive(Error, Debug)]
#[error("{fn}", fn = "...")]
pub struct Error {
    r#fn: &'static str,
}

But in contrast, the following would not compile, due to thiserror using an incorrect comparison and not realizing that a user-provided named argument for {fn} was already present.

#[derive(Error, Debug)]
#[error("{fn}", r#fn = "...")]
pub struct Error {
    r#fn: &'static str,
}

error: duplicate argument named `r#fn`
 --> src/main.rs:4:9
  |
4 | #[error("{fn}", r#fn = "...")]
  |         ^^^^^^  ---- previously here
  |         |
  |         duplicate argument

After this PR, both compile.