Add fixes for :write on filestreams that come from os/open

src/core/ev.c

@@ -1838,6 +1838,16 @@ JanetAsyncStatus ev_machine_write(JanetListenerState *s, JanetAsyncEvent event)
             } else
 #endif
             {
+                // File handles in IOCP need to specify this if they are writing to the 
+                // ends of files, like how this is used here.
+                // If the underlying resource doesn't support seeking
+                // byte offsets, they will be ignored
+                // but this otherwise writes to the end of the file in question
+                // Right now, os/open streams aren't seekable, so this works.
+                // for more details see the lpOverlapped parameter in
+                // https://docs.microsoft.com/en-us/windows/win32/api/fileapi/nf-fileapi-writefile
+                state->overlapped.Offset = (DWORD) 0xFFFFFFFF;
+                state->overlapped.OffsetHigh = (DWORD) 0xFFFFFFFF;
                 status = WriteFile(s->stream->handle, bytes, len, NULL, &state->overlapped);
                 if (!status && (ERROR_IO_PENDING != WSAGetLastError())) {
                     janet_cancel(s->fiber, janet_ev_lasterr());

test/suite0009.janet

@@ -105,6 +105,18 @@
   (file/close outfile)
   (os/rm "unique.txt"))
 
+# Ensure that the stream created by os/open works
+
+(assert-no-error "File writing 4.1"
+   (def outstream (os/open "unique.txt" :wct))
+   (defer (:close outstream)
+     (:write outstream "123\n")
+     (:write outstream "456\n"))
+   # Cast to string to enable comparison
+   (assert (= "123\n456\n" (string (slurp "unique.txt"))) "File writing 4.2")
+   (os/rm "unique.txt"))
+     
+
 # ev/gather
 
 (assert (deep= @[1 2 3] (ev/gather 1 2 3)) "ev/gather 1")