Iterate over syn's structs fields

[sagi21805]

Implementing custom traits on syn's structs is very useful, and sometimes those function could be really repeatable.

if there was an iterator over the names of the fields of syn's structs, it could help creating a macro for those function, so they won't be repeated.

If it is in the project will, I would like to work on this, hopefully it would behave like this:

#[derive(IterFields)]
struct Example {
    a: u32,
    b: u32,
    c: u32,
}

impl IterFields for Exmple {
    const FIELDS: [&'static str; 3] = ["a", "b", "c"]
}