[Breaking change] Remove public exports of gl_common and libc

[tomaka]

cc brendanzab/gl-rs#359

Close #650

[SSheldon]

Removing c_void seems good, but I will mention that *const () is not a C type, so you'll get a warning if you try to use it with an extern function.

I'm not sure if that's a concern here; if it it, the easiest way around it seems to be to make a no-variant enum and use that instead of (), like I did in SSheldon/rust-block@a1c5ef6.

[tomaka]

After checking crates.io, it turns out that absolutely nobody uses that load thing, so I'm tempted to publish a minor version after this change instead of a major version.

[mitchmindtree]

🙏

[mitchmindtree]

🎉

[SimonSapin]

Out of curiosity, why () rather than std::os::raw::c_void?

[tomaka]

@SimonSapin Because I didn't know that existed -_-