Create EventType for RStream reading

RStream will be the main way Neovim receives asynchronous messages, so it is
best to have a specialized EventType for it. A new flag parameter was added to
`rstream_new` which tells the RStream instance to defer event handling for later
with KE_EVENT instead of handling it directly from libuv callback.

src/os/event.c

@@ -8,6 +8,7 @@
 #include "os/event.h"
 #include "os/input.h"
 #include "os/signal.h"
+#include "os/rstream.h"
 #include "os/job.h"
 #include "vim.h"
 #include "memory.h"
@@ -112,6 +113,9 @@ void event_process()
       case kEventJobActivity:
         job_handle(event);
         break;
+      case kEventRStreamData:
+        rstream_read_event(event);
+        break;
       default:
         abort();
     }