Improve performance of decoder's strip function

The strip function in the decoder is a bit of a hotspot. With a small
cpu and a big pipe, everything counts.

Some timings on dev system:
From ~.08sec for 30k element list to ~.0008sec
From ~2sec for 150k element list to ~.004sec
Use zlib's crc32 instead of binascii's in decoder

zlib.crc32() is faster. Shouldn't matter for most, as this is
used in the "no python-yenc" path.