std.os.linux: rework io_uring support

* `linux.IO_Uring` -> `linux.IoUring` to align with naming conventions.
* All functions `io_uring_prep_foo` are now methods `prep_foo` on `io_uring_sqe`, which is in a file of its own.
* `SubmissionQueue` and `CompletionQueue` are namespaced under `IoUring`.

This is a breaking change.

The new file and namespace layouts are more idiomatic, and allow us to
eliminate one more usage of `usingnamespace` from the standard library.
2 remain.

Author: mlugg

CMakeLists.txt

@@ -291,7 +291,8 @@ set(ZIG_STAGE2_SOURCES
     "${CMAKE_SOURCE_DIR}/lib/std/os/linux/errno/generic.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os/linux/x86_64.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os/linux.zig"
-    "${CMAKE_SOURCE_DIR}/lib/std/os/linux/io_uring.zig"
+    "${CMAKE_SOURCE_DIR}/lib/std/os/linux/IoUring.zig"
+    "${CMAKE_SOURCE_DIR}/lib/std/os/linux/io_uring_sqe.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os/linux/x86_64.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os/windows.zig"
     "${CMAKE_SOURCE_DIR}/lib/std/os/windows/ntstatus.zig"

lib/std/os/linux.zig

@@ -383,8 +383,6 @@ pub const O = switch (native_arch) {
     else => @compileError("missing std.os.linux.O constants for this architecture"),
 };
 
-pub usingnamespace @import("linux/io_uring.zig");
-
 /// Set by startup code, used by `getauxval`.
 pub var elf_aux_maybe: ?[*]std.elf.Auxv = null;
 
@@ -4188,22 +4186,9 @@ pub const IORING_SETUP_SINGLE_ISSUER = 1 << 12;
 pub const IORING_SETUP_DEFER_TASKRUN = 1 << 13;
 
 /// IO submission data structure (Submission Queue Entry)
-pub const io_uring_sqe = extern struct {
-    opcode: IORING_OP,
-    flags: u8,
-    ioprio: u16,
-    fd: i32,
-    off: u64,
-    addr: u64,
-    len: u32,
-    rw_flags: u32,
-    user_data: u64,
-    buf_index: u16,
-    personality: u16,
-    splice_fd_in: i32,
-    addr3: u64,
-    resv: u64,
-};
+pub const io_uring_sqe = @import("linux/io_uring_sqe.zig").io_uring_sqe;
+
+pub const IoUring = @import("linux/IoUring.zig");
 
 /// If sqe->file_index is set to this for opcodes that instantiate a new
 /// direct descriptor (like openat/openat2/accept), then io_uring will allocate