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.

2 files changed, 11 insertions, 4 deletions

diff --git a/src/nvim/msgpack_rpc/channel.c b/src/nvim/msgpack_rpc/channel.c
index b4f39c4c61..388fa2584c 100644
--- a/src/nvim/msgpack_rpc/channel.c
+++ b/src/nvim/msgpack_rpc/channel.c
@@ -360,6 +360,17 @@ free_ret:
   api_clear_error(&error);
 }
 
+bool rpc_write_raw(uint64_t id, WBuffer *buffer)
+{
+  Channel *channel = find_rpc_channel(id);
+  if (!channel) {
+    wstream_release_wbuffer(buffer);
+    return false;
+  }
+
+  return channel_write(channel, buffer);
+}
+
 static bool channel_write(Channel *channel, WBuffer *buffer)
 {
   bool success;
diff --git a/src/nvim/msgpack_rpc/unpacker.c b/src/nvim/msgpack_rpc/unpacker.c
index 419349e74d..26c1843026 100644
--- a/src/nvim/msgpack_rpc/unpacker.c
+++ b/src/nvim/msgpack_rpc/unpacker.c
@@ -11,10 +11,6 @@
 # include "msgpack_rpc/unpacker.c.generated.h"
 #endif
 
-#define kv_fixsize_arena(a, v, s) \
-  ((v).capacity = (s), \
-   (v).items = (void *)arena_alloc(a, sizeof((v).items[0]) * (v).capacity, true))
-
 Object unpack(const char *data, size_t size, Error *err)
 {
   Unpacker unpacker;