Added len() function

[Vardan2009]

Closes #120

Now all objects that have len() defined can be called with len(), even with classes!

class Test
{
    fun __constructor__(){};
    fun __len__()
    {
        return 10
    }
}
len(Test()) # 10
len("test") # 4
len([1,2,3]) # 3

[Almas-Ali]

Some issue is raising at tests/in-op.rn L61

[Vardan2009]

The problem seems to be when __contains__(friend_name) returns false, when I change in-op.rn L50 to return true, elt is not None anymore

[Almas-Ali]

The problem seems to be when __contains__(friend_name) returns false, when I change in-op.rn L50 to return true, elt is not None anymore

@angelcaru Can you check this issue?

[Almas-Ali]

I think this feature is complete now. We will fix the test later with another PR.