Remove PartialArray<T> in favor of Span<T> and avoid struct boxing

[h3xds1nz]

Description

Removes the internal PartialArray<T> which is basically duplicating Span<T> functionality (in cases where it is used, not globally, obviously). Since this is then passed to IList<T> params and used as an interface, the struct needs to be boxed on top of everything and most of those calls are done via dispatch instead of using it directly.

Given how few places it is used, we can just replace it and remove the type altogether. In some places it was just used to wrap a regular array without any benefit.

The removed null-checks were really for null but I've checked the call chains and there's no way it should ever be null (or it wouldn't matter if the Span<T> was empty).

Customer Impact

Increased performance, decreased allocations.

Regression

No.

Testing

Local build.

Risk

Low.

Microsoft Reviewers: Open in CodeFlow