Initial support for callbacks

[Smit-create]

Fixes #1608

[certik]

Something like that. I am refactoring the callbacks right now in LFortran. In LFortran, we either specify the function argument to be a Function symbol (instead of a Variable), or use a Variable, see lfortran/lfortran#1578 for a roadmap.

For now I think this PR is fine.

[Smit-create]

we either specify the function argument to be a Function symbol (instead of a Variable)

Yep, I have done the same thing in this PR. As in Fortran, we have an interface, for Python, whenever there's a function as a parameter, I create an empty callback function symbol in the scope of the parent function that works the same way as we do in the Fortran interface.

[certik]

I think that this is fine. Note that we have greatly improved the callbacks in LFortran just yesterday. After merging, do you think you could please sync LFortran/LPython on this?

Another issue: in LFortran we currently allow two ways to do callbacks:

• Var points to Function directly
• Var points to Variable, which points to FunctionType and Function

The second approach I think is more natural and versatile, so we should use it. That requires to sync LFortran's improvements here.

So to move forward, I suggest we merge this, then sync, and then rework it, maybe in few weeks after we harden the new approach in LFortran more.

[Smit-create]

Sure, would rework this once we sync libasr. Thanks!