perf: replace fixed length List with Span

[TimothyMakkison]

Replaces List<string> with Span<string?>, modern collection expressions will generate a fixed length struct, null initialise it and then cast into Span<string?>.

This trick could probably be used in other areas, along with ValueListBuilder and ValueStringBuilder.

Not sure if it's worth using Debug.Assert to ensure that elements contains maxElements + 1 values, I don't see TUnit using it very much.