Reserve space for additional instances in binary deserializer

[kennethloeffler]

This PR adds a new method WeakDom::reserve that can preallocate additional space for instances, and makes the binary deserializer use it before adding instances.

On my machine, this improves rbx_binary's "Deserialize 10,000 Parts" benchmark by ~4%, reduces the number of heap allocations by a negligible amount, but allocates ~2.4MB less memory.

I'm not sure if this should also call reserve on WeakDom.unique_ids, because UniqueIds won't always be present in inputs, and it could take a possibly significant amount of space for no reason.

[Dekkonot]

If we don't reserve extra space in unique_ids it'll make deserializing a place file have a fair few allocations still. This optimization is still worth doing but it's something to keep in mind.

[kennethloeffler]

It doesn't seem to make much a difference for a 6MB rbxl file on my machine, but I guess it won't hurt, since truly massive rbxm files aren't very common anyway

[Dekkonot]

Hilariously for Adopt Me (65MB), it seems to actually make it take longer to deserialize with the UniqueId reservation. If I went through the trouble of checking our full Rojo project it might be more dramatic, since we have around 6000 models with UniqueId written in them, but that's work I don't feel like doing right now.

The difference for Adopt Me is like 11%, so we may want to just not bother with UniqueId reservation.

[kennethloeffler]

Well, that's a bit weird! 🤔