Zig 0.16 Compatibility

[francisqureshi]

I have been using zig-objc for my own Zig 0.16 projects for some time as I've wanted the newer Zig features. I saw that Ghostty looks to be getting ready for Zig 0.16 too, so here is my PR dealing with the following Zig 0.16 changes, I would suggest making a 0.16 branch for this to live on?

So the biggy is @type was removed in Zig 0.16. Zig type construction now uses the new dedicated builtins: @fn, @struct, @tuple, @int etc

Changes:

• Migrate @type to 0.16 builtins (@fn, @struct, @tuple)
• Replace std.io.Writer.Discarding with std.fmt.count
• sliceTo(ptr, 0) → span(ptr) for sentinel-terminated C strings
• Update build.zig for 0.16 API (root_module.link*, getSdk takes b.graph.Io)
• Unwrap wrapper types (Object, Class, Sel) to C types for @fn callconv(.c) compatibility
• Add comptime ABI validation in unwrapType
• Add test for wrapper type args in msgSend

Bonus as its there...

• (fix block.zig: descCopyHelper parameters are swapped. #28) descCopyHelper swapped dst/src params, and added a regression test

When I was initially getting the 0.16 compatibility to work, I have used Claude Code via Sonnet and Opus 4.6 to do the bulk of the work, but I have since reviewed all the code and subsequently worked on it manually, exploring all the options around @type's removal.