Use extension name as library name, instead of hardcoded 'native.so'

[timkpaine]

Opening this PR for some conversation, I'm hoping to distribute an extension via CFFI, but it's a bit awkward integrating it with other projects when the library name is always native.so. It also makes it difficult to link against multiple.

In this PR I've just slightly tweaked the generation bits to make the library name the same as the extension name, so instead of native.so it will have the same name as the outer extension folder. I'm tempted to make it lib<extension_name>.so as it's not an importable python module, so seems like it should use the standard library prefix?

partially fixes #1919

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[messense]

Looks fine to me.

[messense]

I'm tempted to make it lib<extension_name>.so as it's not an importable python module, so seems like it should use the standard library prefix?

IMO it's not a big issue.