Prototype: Directly call parent node's function

[nikhil96sher]

Initial prototype of directly calling parent node's function instead of passing data through channels.

Things currently working:

1. Can directly call parent node's function instead of passing data through channel.
2. Can perform this with multiple downstream nodes in parallel.

Things to add:

1. Eager/Lazy option when creating ExecutionNode. Eager execution node starts the execution in another thread. Lazy execution node doesn't start any execution.
2. Interior Mutability while keeping ExecutionNode immutably borrowed. Important because the same node needs to be borrowed in child nodes. So, we cannot have it borrowed mutably.