Introduce hax_lib::BACKEND::replace_body attribute

[W95Psp]

This PR:

• introduces a replace_body for each backend, that helps rewritting only the body of a function. That's what I use for my SIMD models, that's very handy.
• refactor a bit the proc_macros module in hax_lib so that it's mostly generated by build.rs. This will help to maintain the library.

Example:

    #[hax_lib::fstar::replace_body("magic ${x}")]
    fn f(x: u8, y: u8) -> u8 {
        1 + 2
    }
    struct Foo;
    impl Foo {
        #[hax_lib::fstar::replace_body("(magic (${self} <: $:{Self})) ${x}")]
        fn assoc_fn(&self, x: u8) {}
    }
    impl ToString for Foo {
        #[hax_lib::fstar::replace_body(r#""The type was $:{Self}""#)]
        fn to_string(&self) -> String {
            "Hello".into()
        }
    }

[W95Psp]

@jschneider-bensch this PR might be interesting for you!

[maximebuyse]

That seems useful!