Implement non-copying result access

[zmj]

The current query execution is basically:

foreach (var row in query.Execute) {
    row.AssignTo(out var result); // copy from SQLite memory to managed memory
    // do anything with result, including putting it on the heap
}

This can be optimized for callers that only use the result on the stack. Maybe:

query.Execute.ForEach(result => StackWork(result)) // no copy, Span fields allowed

Where result is constrained to ref struct?

[zmj]

Generics over spans / ref structs aren't currently possible: dotnet/csharplang#1148