Detect duplicate field names after rename attributes are applied

[dtolnay]

This should be an error from serde_derive rather than an unreachable pattern warning from rustc:

struct S {
    a: u8,
    #[serde(rename = "a")]
    b: u8,
}

@cholcombe973 hit this in https://gist.github.com/cholcombe973/a7bf280b1cd5b9cc941cd3e7ab16697a. You have two fields called "fop.fsetattr_count".

[Kixunil]

This would be really helpful. I just almost got bit by a similar problem: two fields renamed to the same name due to bad copy-paste.

[dtolnay]

Closing because I don't plan to pursue this as a priority.