This patch removes the '_as_parameter_' public
attribute of ctypes instances, and replaces the
mechanism by an internal one.

This mechanism is used to convert a ctypes instance to
an (internal) PyCArgObject instance which can directly
by used as an argument in a C function call.

With this patch, a C function pointer which does create
the PyCArgObject instance is stored in the type
dictionary (an StgDict instance).

This does speed up foreign function calls with one
ctypes argument by about 20%, but more important, it
will allow to fix the '_as_parameter_' mechanism, which
is documented in [1], so that it actually will work as
describes, even for functions that have 'argtypes' set.

[1] 
http://docs.python.org/dev/lib/ctypes-calling-functions-with-own-custom-data-types.html

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This patch enables bug fix for the _as_parameter_ mechanism.  Please see 
http://python.org/sf/1533481.  

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The previous patch was against the ctypes repository.  I've
deleted this patch and attached a new one against the Python
svn trunk.

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Please check this in ASAP and remember to make an entry in
Misc/NEWS.  Also, PyObject_stgdict() can return NULL, but
the values aren't checked.  There may be a few other
unchecked returns.

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A variant of this patch together with the general idea of
the patch in #1533481 was committed as SVN rev. 51227.