feat: Instance class

[TechHog8984]

Instances can be created using Instance.new(classname: string) as well as Instance.fromExisting(other: Instance) (note differences between Instance:Clone()).
https://create.roblox.com/docs/reference/engine/classes/Instance

All instances should be refd.

I don't know how I feel about implementing Object as well.

[TechHog8984]

I have heard that Roblox uses a lazy cache for refing, so that is what we will do as well.

[TechHog8984]

The foundation for Instances are pretty much in place; there's just a lack of synchronization plus concern about using shared_ptr everywhere.
Once we handle synchronization and give that concern some thought, we can close this issue.

[TechHog8984]

I've deployed mutexes throughout the code and synchronization is not an issue, to the best of my knowledge.

[TechHog8984]

Another glaring issue I forgot to mention is a lack of default values, namely for Enum values. No Enum members have a default value, which causes a runtime error when indexing.
fixed

[TechHog8984]

Instance:Clone and Instance.fromExisting have been fully implemented

[TechHog8984]

TODO: ValueBase.Changed (different arguments when firing Changed on an instance that is a ValueBase)