Implement `Eq` and `Default` for `ScalarBuffer`

[emilk]

I want to make ScalarBuffer slightly more ergonomic to use with these addition:

• impl Default for ScalarBuffer (e.g. using vec![].into()).
• impl Eq for ScalarbBuffer<T> where T: Eq {}

If it sounds good, I'll make a PR or two

[jayzhan211]

I think Eq makes sense to me, but I have no idea how would Default be useful.

[emilk]

Default is useful so that a struct that owns a ScalarBuffer can have #[derive(Default)] on it

[tustvold]

I don't see any issues with this

[kylebarron]

Happy to make a follow up issue but I'd also love Eq and Default to be implemented on OffsetBuffer, which should be a simple derive because it's defined as

pub struct OffsetBuffer<O: ArrowNativeType>(ScalarBuffer<O>);

[emilk]

I think that makes sense, but I'm gonna close this issue as completed