Fix compilation on Zig 0.16

[lolzdev]

I chaged @Type to @Fn and @Struct, fixed build.zig, updated the code to the new std.Io and did some other minor things to make everything work on 0.16.

[Uzaaft]

zig-objc only works with released versions of Zig. We don't support nightly versions because the Zig compiler is still changing too much.

This probably wont land until 0.16 is out.

[loftafi]

Thanks for sharing a 0.16 compatible patch.