Hook in custom callbacks to ccall

[certik]

In pure Python mode we need some way to register custom callbacks with ltypes, for example:

import ltypes
def callback(name):
    ...
ltypes.register_ccall_callback(callback)

@ccall
def f(..)
   ...

Now when f is called in pure Python mode, ltypes that implements the ccall decorator would instead call the user defined callback function that can provide custom callback, so that we do not need to call into C.

[certik]

This was implemented in https://github.com/lcompilers/lpython/pull/1740/files#diff-851646ee31435bb9b3ae29b961902cf243cb91b3b145ef2e3df7e095b47150deR288, but we don't have a test. Here is how to use it:

import my_implementations
import lpython
from types import FunctionType
lpython.CTypes.emulations = {k: v for k, v in my_implementations.__dict__.items()
        if isinstance(v, FunctionType)}

Here the my_implementations module contains regular Python functions (instances of FunctionType) that provide a pure Python implementations of the ccalls. We assign to the emulations class variable.