Sum, Product implementations for Iterator<Vector*>

[jamesmcm]

Add implementations for std::iter::Sum and std::iter::Product for the Vector (and IVector) types.

Note both operations are element-wise (this comes from glam).

Due to there being multiple Add and Mul implementations (ultimately in glam), I noticed having to specify types explicitly in some circumstances. I imagine this can probably be improved but I am not an expert on generic programming.

[GodotRust]

API docs are being generated and will be shortly available at: https://godot-rust.github.io/docs/gdext/pr-666

[fpdotmonkey]

There also should be a docstring explaining this is all element-wise since that's not obvious for Product. Not that dot- or cross-product would make any sense for this, but those are what I'd think of first as being Vector * Vector multiplication.

Presently, there is no impl Mul<Vector> for Vector. I think that should be added since it would be incongruous to have element-wise multiplication for iterators, but not values.

Also no need to qualify that glam does element-wise multiplication; Godot does as well.

[jamesmcm]

Thanks, I've made these changes.

impl Mul<Vector> for Vector exists already - I think it was confusing due to the existing vector_vector macro still accepting the $Scalar value before even though it was unused, so I've removed that here too.

[Bromeon]

Thank you, and congrats to your first merged PR! 🚀