Allow monkey patching on builtin types

[cgccuser]

Currently, if you try something like str.foo = 3, you get this:

Traceback (most recent call last):
  File ".code.tio", line 1, in <module>
    str.foo = 3
TypeError: can't set attributes of built-in/extension type 'str'

This "feature" may or may not be extremely hard to implement.