This makes the test runs in pure_python mode 15s faster.

Microbenchmarks on CPython 2.7.14:

$ python -m perf timeit -s 'from struct import pack,unpack' 'unpack("i", pack("i", 123456))'
.....................
Mean +- std dev: 365 ns +- 12 ns

$ python -m perf timeit -s 'from struct import Struct; s = Struct("i"); unpack=s.unpack; pack=s.pack' 'unpack(pack(123456))'
.....................
Mean +- std dev: 251 ns +- 9 ns

No appreciable difference on PyPy.