Add file and pipe Windows wrappers

This includes Reader, Writer, and Closer interfaces on top of
Windows file handles in a way that does not block the system thread
while the IO is outstanding.

This also includes net.Listener and net.Conn for named pipes.

syscall.go

@@ -0,0 +1,3 @@
+package winio
+
+//go:generate go run mksyscall_windows.go -output zsyscall.go file.go pipe.go