job: Replace by a better process abstraction layer

- New libuv/pty process abstraction with simplified API and no globals.
- Remove nvim/os/job*. Jobs are now a concept that apply only to programs
  spawned by vimscript job* functions.
- Refactor shell.c/channel.c to use the new module, which brings a number of
  advantages:
  - Simplified API, less code
  - No slots in the user job table are used
  - Not possible to acidentally receive data from vimscript
- Implement job table in eval.c, which is now a hash table with unilimited job
  slots and unique job ids.

src/nvim/os_unix.c

@@ -50,7 +50,6 @@
 #include "nvim/os/input.h"
 #include "nvim/os/shell.h"
 #include "nvim/os/signal.h"
-#include "nvim/os/job.h"
 #include "nvim/msgpack_rpc/helpers.h"
 
 #ifdef HAVE_STROPTS_H