Allow AsTraitPaths in arithmetic generics

[jfecher]

Problem

As mentioned in #5739, AsTraitPaths (<T as Trait>::Type) currently aren't allowed in arithmetic generics.

Happy Case

We should allow these in arithmetic generics to allow for code like:

impl<A, B> Serialize for (A, B) where A: Serialize, B: Serialize {
    let Size = <A as Serialize>::Size + <B as Serialize>::Size;
    ...
}

Once we allow for implicit associated types as well.

Workaround

These generics can be made explicit instead:

impl<A, B, let AS: u32, let BS: u32> Serialize for (A, B) where A: Serialize<Size = AS>, B: Serialize<Size = BS> {
    let Size = AS + BS;
    ...
}

Workaround Description

No response

Additional Context

No response

Project Impact

None

Blocker Context

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Support Needs

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