Add attrs() and attrs_mut() getters to syn::Item

[Robbepop]

While developing certain proc macros using syn I found getters that return the attribute slice for a generic syn::Item to be missing. The implementation is straight forward and simple, however, daunting:

Example through some extension trait

impl Attrs for syn::Item {
    fn attrs(&self) -> &[syn::Attribute] {
        use syn::Item;
        match self {
            Item::Const(syn::ItemConst { attrs, .. })
            | Item::Enum(syn::ItemEnum { attrs, .. })
            | Item::ExternCrate(syn::ItemExternCrate { attrs, .. })
            | Item::Fn(syn::ItemFn { attrs, .. })
            | Item::ForeignMod(syn::ItemForeignMod { attrs, .. })
            | Item::Impl(syn::ItemImpl { attrs, .. })
            | Item::Macro(syn::ItemMacro { attrs, .. })
            | Item::Macro2(syn::ItemMacro2 { attrs, .. })
            | Item::Mod(syn::ItemMod { attrs, .. })
            | Item::Static(syn::ItemStatic { attrs, .. })
            | Item::Struct(syn::ItemStruct { attrs, .. })
            | Item::Trait(syn::ItemTrait { attrs, .. })
            | Item::TraitAlias(syn::ItemTraitAlias { attrs, .. })
            | Item::Type(syn::ItemType { attrs, .. })
            | Item::Union(syn::ItemUnion { attrs, .. })
            | Item::Use(syn::ItemUse { attrs, .. }) => attrs,
            _ => &[],
        }
    }
}

Note that a getter for this would also have the benefit of such dependent crates to not care if syn ever adds new variants. If syn provided this dependencies could simply make use of syn handling the update.

Would you accept a PR that adds these getters? If so I'd be happy to file a PR.

[dtolnay]

Seems reasonable. Please send a PR.