Add buffer()-compatible function for Python 3 to twisted.python.compat

[itamar]

buffer() is used in twisted.internet.tcp, twisted.internet.abstract (and elsewhere: IOCP and unix.py). We should provide a sufficiently compatible version based on memoryview to meet the requirements of those users.

[exarkun]

But probably not based on memoryview, due to the problems it comes with (complexity, poor documentation, incompatibility with parts of pyOpenSSL, etc).

[itamarst]

(In [36118]) Branching to 'buffer-py3-6099'

[itamar]

 http://buildbot.twistedmatrix.com/boxes-supported?branch=/branches/buffer-py3-6099 is running.

Note that the argument names were based on the Python 2 buffer() argument names -  http://docs.python.org/library/functions.html#buffer

[exarkun]

Regarding naming... How about a name that doesn't give the appearance of being more than trivially buffer-compatible - lazyByteSlice or something descriptive like that? The idea behind this being that our requirement is not for a buffer replacement (and this isn't one, since that would be a ton of work), but for a way to slice strings without copying memory unnecessarily. buffer did that for us, but is kind of an implementation detail.

Alternatively (and I think this is worse), be more explicit in documenting the limitations of our buffer.

Otherwise this looks great. Please address the naming point somehow and then merge.

[itamarst]

(In [36128]) Merge buffer-py3-6099.

Author: itamar Review: exarkun Fixes: #6099

Add function to emulate buffer() on both Python 2 and (less efficiently) on Python 3.