Allow per-item configuration

Related to https://github.com/rust-lang/rustfmt/issues/3989#issuecomment-570737505.

Rationale: I have some code that looks like

enum InternalDeclarator {
    Id(InternedStr),
    Pointer { qualifiers: Vec<DeclarationSpecifier> },
    Array { size: Option<Box<Expr>> },
    // TODO: support abstract parameters
    Function { params: Vec<(TypeName, InternedStr)>, varargs: bool },
}

I'd like it to stay the same (i.e. all on one line) by setting use_small_heuristics = "Max". However, that changes all the code in the rest of the repository as well. Right now, I'm just using #[rustfmt::skip]. Could rustfmt add a way to set this for individual items (or even variants)?

I was imagining it would look something like this:

#[rustfmt::use_small_heuristics = "Max"]
enum InternalDeclarator {
    Id(InternedStr),
    Pointer { qualifiers: Vec<DeclarationSpecifier> },
    Array { size: Option<Box<Expr>> },
    // TODO: support abstract parameters
    Function { params: Vec<(TypeName, InternedStr)>, varargs: bool },
}

Start with the related discussion at rust-lang/rustfmt#3989 and compare the requested per-item setting with the existing #[rustfmt::skip] approach and the global use_small_heuristics = "Max" setting. The scope needs to settle whether attributes apply to items, variants, or both; done means a defined and implemented way to override the heuristic locally without changing unrelated code.