integrate std.os.write and equivalent with evented I/O

[andrewrk]

The main change to do is:

-            EAGAIN => unreachable, // This function is for blocking writes.
+            EAGAIN => if (std.event.Loop.instance) |loop| {
+                loop.waitUntilFdWritable(fd);
+                continue;
+            } else {
+                return error.WouldBlock;
+            },

And then fix everything that breaks. This involves changes to std.io.OutStream, and unfortunately std.fmt.format. I was going to do this change as part of the std.net branch (#3538) but it got tricky, so I'm opening a separate issue for it.

[frmdstryr]

Well this opens a can of worms. I went down the rabbit hole attempting this by converting out_stream and std.fmt and hit #3157 where the thread's startFn async but can't be because it's an extern.

[andrewrk]

Done in #4404, landed in 014f66e.