Fix up scalar values in Vortex

[gatesn]

Vortex scalars are currently very heavy-weight objects. They're not necessarily on the hot path in many places, but they come with a lot of baggage.

As arrays move towards becoming logical plans, the Array::scalar_at function will need to perform compute. Scalars in their current form are also very inefficient for nested types such as structs and lists, requiring each child to be its own Arc value.

Proposal

• Scalars become a physical concept. In other words, they no longer have a DType.
• We remove the Scalar struct entirely, keeping only ScalarValue.
• ScalarValue::List holds an ArrayRef representing the elements in the list scalar.
• ScalarValue::Struct holds a Vec for each field.
• We remove the vortex-scalar crate, moving ScalarValue into vortex-array (since it now references ArrayRef)

Questions

• Do we keep Array::scalar_at? Or do we move it to Canonical::scalar_at? Or do we require users to Array::slice(..).execute:: or similar?
• If it's Canonical::scalar_at, it's awkward that validity isn't also canonicalized.
• If it's execution, then is slicing too expensive? Perhaps not? What about filter with a length-1 mask?

[robert3005]

I think if Scalars can hold single element arrays then we can have Array::scalar_at but we should mention in docs that repeatedly calling it is not the optimal behaviour (but getting single element is).