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/misc1.c

@@ -61,6 +61,7 @@
 #include "nvim/os/shell.h"
 #include "nvim/os/input.h"
 #include "nvim/os/time.h"
+#include "nvim/event/stream.h"
 
 #ifdef INCLUDE_GENERATED_DECLARATIONS
 # include "misc1.c.generated.h"