Add channel module

- Add channel module that exposes the API over arbitrary streams
- Add `xmemdup` for duplicating memory chunks
- Make job exit callback optional

src/os/event.c

@@ -7,6 +7,7 @@
 #include "lib/klist.h"
 #include "os/event.h"
 #include "os/input.h"
+#include "os/channel.h"
 #include "os/signal.h"
 #include "os/rstream.h"
 #include "os/job.h"
@@ -36,6 +37,8 @@ void event_init()
   signal_init();
   // Jobs
   job_init();
+  // Channels
+  channel_init();
   uv_timer_init(uv_default_loop(), &timer);
   // This prepare handle that actually starts the timer
   uv_prepare_init(uv_default_loop(), &timer_prepare);
@@ -43,6 +46,7 @@ void event_init()
 
 void event_teardown()
 {
+  channel_teardown();
   job_teardown();
 }