Support for packed structs

[dagss]

Could we have some kind of keyword to define packed structs in Cython? This is not part of the C standard, but is supported in most C compilers through extensions. (Obviously one can do it now through writing a header file, but that is inconvenient.)

I.e. something like:

cdef packed struct MyStruct:
     char a
     double b

would output conditional code to do either attribute((packed)) or #pragma pack(push,1), #pragma pack(pop) (gcc/icc and Visual C, respectively).

[dagss]

It turns out that #pragma pack is supported on GCC >= 3.0. However I can spot at least one instance of Python 2.3 on GCC 2.95 on the internet so I'll include the GCC method as well.

[dagss]

 http://hg.cython.org/cython-devel/rev/a47a02f32eba