Handle NUL bytes in strings everywhere

[andychu]

From #387

This probably happens all over the place. Options:

1. silently truncate (bash)
2. raise a fatal error

I think I want to get rid of the concept of "warnings".

It's similar what happens when we have invalid UTF-8.

[andychu]

Note: we have an issue where CPython will raise exceptions on internal NUL bytes, but our C++ bindings don't, and probably shouldn't

>>> import os
>>> os.system('ls\0')
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: system() argument 1 must be string without null bytes, not str
>>> 

[andychu]

This works in C++, and is tested