Support skip attribute in Eq/PartialEq derives (#163)

[tyranron]

Part of #163
Requires #479

Synopsis

Support of skip attribute on fields and variants:

#[derive(Eq, PartialEq)]
enum E {
    Foo(bool, #[eq(skip)] NoEq),
    #[partial_eq(skip)]
    Bar(NoEq, NoEq),
}

Solution

Allow utils::attr::Skip on fields and variants.

Ergonomics

#[eq(skip)]/#[partial_eq(skip)] attributes are visible and mean the same for both Eq and PartialEq macros, allowing to use only one and do not repeat yourself.

TODO: describe variant logic

Checklist

• Documentation is updated
• Tests are added/updated
• CHANGELOG entry is added (not required)