rstream/wstream: Unify structures and simplify API

- Simplify RStream/WStream API and make it more consistent with libuv.
- Move into the event loop layer(event subdirectory)
- Remove uv_helpers module.
- Simplify job/process internal modules/API.
- Unify RStream and WStream into a single structure. This is necessary because
  libuv streams can be readable and writable at the same time(and because the
  uv_helpers.c hack to associate multiple streams with libuv handle was removed)
- Make struct definition public, allowing more flexible/simple memory
  management by users of the module.
- Adapt channel/job modules to cope with the changes.

src/nvim/ex_docmd.c

@@ -73,8 +73,8 @@
 #include "nvim/os/time.h"
 #include "nvim/ex_cmds_defs.h"
 #include "nvim/mouse.h"
-#include "nvim/os/rstream.h"
-#include "nvim/os/wstream.h"
+#include "nvim/event/rstream.h"
+#include "nvim/event/wstream.h"
 
 static int quitmore = 0;
 static int ex_pressedreturn = FALSE;