perf(ui): reduce allocation overhead when encoding "redraw" events

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.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/nvim/lib/kvec.h b/src/nvim/lib/kvec.h
index 4238088b1c..b5b3adf7d2 100644
--- a/src/nvim/lib/kvec.h
+++ b/src/nvim/lib/kvec.h
@@ -121,6 +121,9 @@
 #define kv_push(v, x) \
   (*kv_pushp(v) = (x))
 
+#define kv_pushp_c(v) ((v).items + ((v).size++))
+#define kv_push_c(v, x) (*kv_pushp_c(v) = (x))
+
 #define kv_a(v, i) \
   (*(((v).capacity <= (size_t)(i) \
       ? ((v).capacity = (v).size = (i) + 1, \