This module provides a mutable string implementation suitable for use as
a buffer - IOW, we want to be able to do some basic stringish things, but
especially, we want to be able to append to the end and delete from the
beginning in constant time.

It's intended to eventually run on Python 2.x and Python 3.x, and to
provide a consistent API for dealing with buffers between the two.
However, for now the coding is all happening on Python 2.x, as that's
what the author is most familiar with, and that's where pylint works.

Please note that the 2.x version may be a bit slow on Python
interpreters for which "str1 = str1 + str2" is slow.

The representation is a linked list of substrings each of chunk_size
characters or fewer.

It doesn't support all the fancy str operations provided by 2.x str's;
it does however support the common things, especially those needed of a
buffer.

Note that __delitem__ is not yet fully implemented for extended slices
(it needs special casing for step=1, because it could be much faster
with some extra code), and __setitem__ may never be improved to deal
with extended slices.