Currently only merges history when reading ShaDa file. No tests yet.

This is simpler and more efficient than klist.h for implementing queues that
support insertion or removal at arbitrary positions.

- 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.

- Add `kl_shift_at` macro and backing function. This can be used to shift
  elements at arbitrary positions. `kl_shift` is now defined on top of the new
  macro.
- Change shift/push API, now `kl_push` accepts an object as parameter and
  `kl_shift` returns the object instead of a status. An assertion against
  shifting at the end of a list(or empty lists) was added.
- Add `kl_iter` and `kl_iter_at` macros. `kl_iter_at` is for starting the
  iteration at arbitrary positions.

We already use wrappers for allocation, the new `xfree` function is the
equivalent for deallocation and provides a way to fully replace the malloc
implementation used by Neovim.