Feature fqdn entrypoint

[tito]

This add a new option that allow changing the name of the python init symbol in order to bundle a bunch of cythonized python files into a single .so.

I explained the usage of this feature in the Initial post about it https://groups.google.com/forum/#!topic/cython-users/ZVrDYHAnMi8

To resume, this prevent the conflict between the same init name symbol between multiple cythonized files, when thoses files are within a single library (on android), or statically included into the final binary (on iOS, mandatory as dlopen is prohibed).

The option is named use_fqdn_entrypoint, default to False, and is handled as a compiler directive.

If i have a file at kivy/graphics/opengl.pyx, without the option, the entrypoint would be initopengl. With the option activated, it would be initkivy_graphics_opengl. Obviously a custom Python meta-importer is required, but it's up to the user to write it.

[robertwb]

I'd like to have a more principled approach to bundling modules into a single so (or executable), complete with injecting the right import hooks, but that'd take longer to design. As this is a blocker for some platforms, I'm willing to consider this intermediate suggestion.

The name "use_fqdn_entrypoint" isn't very suggestive to me. Perhaps, as it's specified as a directive, let the user choose the full name itself (e.g. initmodule_cname=XXX) I would be open to sentinel values that indicate that the module name or fully qualified module name should be used.

Also, tests?

[scoder]

PEP 489 has a section about multiple modules in one shared library. Since Cython now has support for PEP 489 module initialisation, we should build on that.