Psyco-friendly patch #2.

A very very small statistic-collecting patch.


pystate.h: added a field at the end of the
PyThreadStruct:

    int tick_counter;

ceval.c: eval_frame():

    tstate->tick_counter is incremented whenever the
    check_interval ticker reaches zero.

The purpose is to give a useful measure of the number
of interpreted
bytecode instructions in a given thread.  This
extremely lightweight
statistic collector can be of interest to profilers
(like psyco.jit()).

We can safely guess that a single integer increment
every 100
interpreted bytecode instructions will go entierely
unnoticed
in any performance measure. [This is true for
pystone.py.]