Mobx-state-tree: Introduce optional generic argument in types.frozen (and types.localState) to make it easily strong typeable

[mweststrate]

[mattiamanzati]

New plan: Support types.immutable(Type) which basically froze the node and throw if childs or node is changing

[xaviergonz]

really nice idea, I've got some nodes that are supposed to be immutable once attached to a type but there's no way to enforce it other than making the type itself actionless (e.g points, colors...)

[mweststrate]

This can now be expressed using types.custom as well (which just returns the value in the from/to snapshot functions without further transformation)