Improved Node Graph support for user created nodes.

[adamgerhant]

It should be possible to select a group of nodes, and then "group" them into a single node with a network implementation of those nodes. It should also be possible to change the number of inputs/outputs and customize the input/output names. It could also be useful to "ungroup" a node. Once (#2042) is complete, it should also be possible to change the annotated types for the inputs.

[Keavon]

This will also be an important part of making lambdas more useful and approachable, once we implement them. To produce constructs like loops.