Zig's new Writer

Zig's IO namespace is being reworked with a new Writer interface that includes built-in buffering. The new std.Io.Writer requires implementing a drain method that handles arrays of strings and includes a splat parameter. Writers now need to provide buffers when created, and the interface uses a vtable pattern with fieldParentPtr for implementation. The change aims to improve performance through vectored I/O support but requires migration from existing APIs, with some functions moving from standalone utilities to Writer methods.