Improve casting

[chippmann]

Describe the problem or limitation you are having in your project:
Currently you solution for casting GodotObjects is very limiting and performance hungry.
We need to find a better way of casting objects.

Describe how this feature / enhancement will help you overcome this problem or limitation:
Currently a user cannot cast a node to a custom class which derives from node as it has no from function.

Show a mock up screenshots/video or a flow diagram explaining how your proposal will work:
None to provide

Describe implementation detail for your proposal (in code), if possible:
None to provide

If this enhancement will not be used often, can it be worked around with a few lines of code?:
Not really.

Is there a reason why this should be in this project and not individually solved?:
Yes the casting is implemented in the core project.

[chippmann]

Closed in favor of the merge of the bindings.