Avoid using magic number indexes in the node system

[Keavon]

There are tons of fragile index values like 0, 1, 2, etc. used to reference inputs and outputs (amongst other things?) between nodes and node internals. This is extremely fragile, makes it hard to refactor, provides a difficult developer experience, lacks self-documenting clarity, and is hard to debug.

One potential solution could be using enums. But thinking through some alternative ways to achieve this could also be good.