based on a patch by felix schwarz, thanks!
changes after that:
build msgpack cpp files at release time, do not use cython otherwise.
we want to include .cpp (and .c) files into the sdist, so users
installing the pypi package won't need to have cython installed.
also: python setup.py clean shall also cleanup the *.cpp and binaries
built from that.
.gitignore: ignore generated msgpack .cpp sources
setup.cfg: flake8: ignore 3rd party bundled msgpack source
CRC slice by 8 for generic CPUs outperforms zlib CRC32 on ppc
and x86 (ARM untested but expected to as well).
PCLMULQDQ derived from Intel's zlib patches outperforms every other
CRC implementation by a huge margin.
the generation of those files was causing us way too much pain to
justify automatically generating them all the time.
those will have to be re-generated with `build_api` or `build_usage`
as appropriate, for example when function signatures or commandline
flags change.
see #384
the current instructions create a dirty tree that `git add .` would
commit into git. this is error prone and somewhat unclean.
i found it preferable to change the `.gitignore` than to change the instructions, since there are probably `borg-env` environments lying around everywhere already.