Cache conscientiousness round #2: compact argparse structures

This reduces the overhead for a struct argspec from 24 bytes to 4 bytes,
and removes one test from the positional argument parsing loop.

[11:02:48 ks2!17 py-lmdb] cat old
   text    data     bss     dec     hex filename
  75388    6928     240   82556   1427c lmdb/cpython.so
[11:02:49 ks2!18 py-lmdb] cat new
   text    data     bss     dec     hex filename
  73439    5048      24   78511   132af lmdb/cpython.so
[11:02:51 ks2!19 py-lmdb] bc
bc 1.06.95
Copyright 1991-1994, 1997, 1998, 2000, 2004, 2006 Free Software Foundation, Inc.
This is free software with ABSOLUTELY NO WARRANTY.
For details type `warranty'.
82556- 78511
4045