refactor(ui): devirtualize the ui layer

- The defined interface for the UI is only the RPC protocol. The original
  UI interface as an array of function pointers fill no function.
- On the server, all the UI:s are all RPC channels.
  - ui.c is only used on the server.
  - The compositor is a preprocessing step for single-grid UI:s
- on the client, ui_client and tui talk directly to each other
  - we still do module separation, as ui_client.c could form the basis
    of a libnvim client module later.

Items for later PR:s
- vim.ui_attach is still an unhappy child, reconsider based on plugin experience.
- the flags in ui_events.in.h are still a mess. Can be simplified now.
- UX for remote attachment needs more work.
- startup for client can be simplified further (think of the millisecs we can save)

src/nvim/ui.h

@@ -52,6 +52,41 @@ enum {
 
 typedef int LineFlags;
 
+typedef struct {
+  uint64_t channel_id;
+
+#define UI_BUF_SIZE 4096  ///< total buffer size for pending msgpack data.
+  /// guaranteed size available for each new event (so packing of simple events
+  /// and the header of grid_line will never fail)
+#define EVENT_BUF_SIZE 256
+  char buf[UI_BUF_SIZE];  ///< buffer of packed but not yet sent msgpack data
+  char *buf_wptr;  ///< write head of buffer
+  const char *cur_event;  ///< name of current event (might get multiple arglists)
+  Array call_buf;  ///< buffer for constructing a single arg list (max 16 elements!)
+
+  // state for write_cb, while packing a single arglist to msgpack. This
+  // might fail due to buffer overflow.
+  size_t pack_totlen;
+  bool buf_overflow;
+  char *temp_buf;
+
+  // We start packing the two outermost msgpack arrays before knowing the total
+  // number of elements. Thus track the location where array size will need
+  // to be written in the msgpack buffer, once the specific array is finished.
+  char *nevents_pos;
+  char *ncalls_pos;
+  uint32_t nevents;  ///< number of distinct events (top-level args to "redraw"
+  uint32_t ncalls;  ///< number of calls made to the current event (plus one for the name!)
+  bool flushed_events;  ///< events where sent to client without "flush" event
+
+  int hl_id;  // Current highlight for legacy put event.
+  Integer cursor_row, cursor_col;  // Intended visible cursor position.
+
+  // Position of legacy cursor, used both for drawing and visible user cursor.
+  Integer client_row, client_col;
+  bool wildmenu_active;
+} UIData;
+
 struct ui_t {
   bool rgb;
   bool override;  ///< Force highest-requested UI capabilities.
@@ -65,13 +100,9 @@ struct ui_t {
   double pum_col;
   double pum_height;
   double pum_width;
-  void *data;
 
-#ifdef INCLUDE_GENERATED_DECLARATIONS
-# include "ui_events.generated.h"
-#endif
-
-  void (*inspect)(UI *ui, Dictionary *info);
+  // TODO(bfredl): integrate into struct!
+  UIData data[1];
 };
 
 typedef struct ui_event_callback {