Use GetUninitializedObject in IStructuralReadWrite.Read

[joshua-spacetime]

That is, fix this TODO:

SpacetimeDB/crates/bindings-csharp/BSATN.Runtime/BSATN/Runtime.cs

Lines 14 to 16 in 59faab8

	// TODO: use `RuntimeHelpers.GetUninitializedObject` as an optimisation here.
	// We tried but couldn't do this because BitCraft relies on being able
	// to add and initialize custom fields on autogenerated classes.

Currently each row object is instantiated using reflection during deserialization which represents a significant portion of the total cost as the following profile snippet demonstrates.

[kazimuth]

I didn't realize new T() compiles to something with reflection, that's nasty. Life without monomorphisation I guess.

[joshua-spacetime]

Note, if the above TODO cannot be resolved, an alternative would be to cache new T() and just copy it for each row decode.