Revised implementation of CALL_FUNCTION and friends.

More revision still needed.

Much of the code that was in the mainloop was moved to a series of
helper functions.  PyEval_CallObjectWithKeywords was split into two
parts.  The first part now only does argument handling.  The second
part is now named call_object and delegates the call to a
call_(function,method,etc.) helper.

XXX The call_XXX helper functions should be replaced with tp_call
functions for the respective types.

The CALL_FUNCTION implementation contains three kinds of optimization:
1. fast_cfunction and fast_function are called when the arguments on
   the stack can be passed directly to eval_code2() without copying
   them into a tuple.
2. PyCFunction objects are dispatched immediately, because they are
   presumed to occur more often than anything else.
3. Bound methods are dispatched inline.  The method object contains a
   pointer to the function object that will be called.  The function
   is called from within the mainloop, which may allow optimization #1
   to be used, too.

The extened call implementation -- f(*args) and f(**kw) -- are
implemented as a separate case in the mainloop.  This allows the
common case of normal function calls to execute without wasting time
on checks for extended calls, although it does introduce a small
amount of code duplication.

Also, the unused final argument of eval_code2() was removed.  This is
probably the last trace of the access statement :-).