[LISTS] Add C support for len function

[EmilyBourne]

Describe the feature

In Python the len function can be called on many functions however the implementation of this function is different in different languages. Support should be added to test the length of a list in C (and Python if it does not work out of the box).

Test Code

Provide code which does not currently work but which should do when this issue is fixed:

def f():
    a = [1,2,3]
    b = len(a)
    return b

Proposed Solution

This feature should be implemented using the functionalities provided by STC