Fixes #79

Slightly faster on CPython:

$ python -m perf timeit -s 'from struct import Struct; s = Struct("i"); unpack=s.unpack; pack=s.pack' 'unpack(pack(123456))'
.....................
Mean +- std dev: 252 ns +- 12 ns
$ python -m perf timeit -s 'from struct import Struct; s = Struct("i"); from operator import index; pack=s.pack' 'pack(index(123456))'
.....................
Mean +- std dev: 235 ns +- 6 ns

Very slightly faster on PyPy:

python -m perf timeit -s 'from struct import Struct; s = Struct("i"); unpack=s.unpack; pack=s.pack' 'unpack(pack(123456))'
.........
Mean +- std dev: 4.24 ns +- 0.12 ns
$ python -m perf timeit -s 'from struct import Struct; s = Struct("i"); from operator import index; pack=s.pack' 'pack(index(123456))'
.........
Mean +- std dev: 4.19 ns +- 0.10 ns