Same idea as (but implementation not directly based on) the Python shlex module. However, this implementation does not support any of the Python module's customization because it makes parsing slower and is fairly useless. You only get the default settings of shlex.split, which mimic the POSIX shell: http://pubs.opengroup.org/onlinepubs/9699919799/utilities/V3_chap02.html
This implementation also deviates from the Python version in not treating \r specially, which I believe is more compliant.
The algorithms in this crate are oblivious to UTF-8 high bytes, so they iterate over the bytes directly as a micro-optimization.