I thought it would be nice if mmaps could generally look a little more
like sequences. Specifically, being able to resize+write using
square-bracket notation as with lists:

>>> x = [1,2,3,4,5]
>>> x
[1, 2, 3, 4, 5]
>>> x[2:2] = [6,7,8,9]
>>> x
[1, 2, 6, 7, 8, 9, 3, 4, 5]
>>>

If that could be done when x is an mmap.mmap, it'd be great.
alternatively, if mmap had an insert or an extend method that work like
with lists, the same behavior could be achieved without relying on mmap
specific method-names.