Make the Graphene type system more powerful with HM type inference

[Keavon]

Implement https://en.wikipedia.org/wiki/Hindley–Milner_type_system in Graphene. Essentially this means we'd have up-and-down type inference, like what's common in languages such as TS or Rust. This can help us use the appropriate node implementation based on its surroundings, which is particularly important for dealing with the involvement or non-involvement of Footprint in a type (especially with the Transform node). It'll also let us know which typed implementation of a node is the appropriate one to reference when a node input is disconnected (but it's not part of the rendered graph) and that input has to be set to some type's default value.

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• Type inference at declaration site when usage site is further up or down in the node flow (needed to allow the Number value node to have a built-in Into node)
  • Support for traits