Attributes for ops macros

[50U10FCA7]

Proposal

Add a #[add(types(..))] and #[add(forward)] attributes (the same way it works for From and Into) for Add, AddAssign, Sub, SubAssign, Mul and MulAssign macros

It will help to avoid boilerplate for newtype structs.

Example

#[derive(Add, AddAssign, Mul, MulAssign, Sub, SubAssign)]
#[add(types(usize))]
#[mul(forward)]
#[sub(forward)]
pub struct MyIndex(usize);

Edit1: Fixed typo.

[tyranron]

@50U10FCA7 I guess you've meant #[mul(forward)].