Work on https://github.com/neovim/neovim/issues/459

Co-authored-by: Famiu Haque <famiuhaque@protonmail.com>

Make the copy_object() family accept an optional arena. More than
half of the callsites should be refactored to use an arena later
anyway.

This is both simpler in client code and more effective (always reuse
block hottest in cache)

Problem:    It is not possible to specify cell widths of characters.
Solution:   Add setcellwidths().
https://github.com/vim/vim/commit/08aac3c6192f0103cb87e280270a32b50e653be1

Co-Authored-By: delphinus <me@delphinus.dev>

In case nvim A sends nvim_error_event to nvim B, it would
respond with another nvim_error_event due to unknown
request name. Fix this by adding dummy request handler for now.

Problem:    Get ml_get error when deleting a line in 'completefunc'. (Yegappan
            Lakshmanan)
Solution:   Lock the text while evaluating 'completefunc'.
https://github.com/vim/vim/commit/ff06f283e3e4b3ec43012dd3b83f8454c98f6639

Fix a mistake in the porting of patch 8.1.0098.
Cherry-pick Test_run_excmd_with_text_locked() from patch 8.2.0270.
Cherry-pick test_gf.vim changes from patch 8.2.0369.
Cherry-pick message change from later patches.

Note for external UIs: Nvim can now emit multiple "redraw" event batches
before a final "flush" event is received. To retain existing behavior,
clients should make sure to update visible state at an explicit "flush"
event, not just the end of a "redraw" batch of event.

* Get rid of copy_object() blizzard in the auto-generated ui_event layer
* Special case "grid_line" by encoding screen state directly to
  msgpack events with no intermediate API events.
* Get rid of the arcane notion of referring to the screen as the "shell"
* Array and Dictionary are kvec_t:s, so define them as such.
* Allow kvec_t:s, such as Arrays and Dictionaries, to be allocated with
  a predetermined size within an arena.
* Eliminate redundant capacity checking when filling such kvec_t:s
  with values.

feat: user command "preview" (like inccommand)