dev: be coherent about math trait taking a snap or the value itself

[tdelabro]

Here is PartialEq trait definition:
https://github.com/starkware-libs/cairo/blob/main/corelib/src/traits.cairo#L55-L60

For some reason, the methods of this trait take their arg as snapshot, while all other trait takes them as value:

• PartialOrd https://github.com/starkware-libs/cairo/blob/main/corelib/src/traits.cairo#L86-L91
• Add https://github.com/starkware-libs/cairo/blob/main/corelib/src/traits.cairo#L10-L12
  and so on

Is there a reason for this special treatment?
We should be coherent across the core lib so I suggest changing the trait definition to

pub trait PartialEq<T> {
    fn eq(lhs: T, rhs: T) -> bool;
    fn ne(lhs: T, rhs: T) -> bool {
        !Self::eq(lhs, rhs)
    }
}

I don't know what it implies in terms of breaking changes. Maybe those can be mitigated from a dev perspective by the use of implicit deref...