- increase python line-length limit from 88 => 100.
- gen_help_html: fix bug in "tag" case (tbl_count => tbl_contains)

ref #15632
fix #18215
fix #18479
fix #20527
fix #20532

Co-authored-by: Ben Weedon <ben@weedon.email>

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

This introduces the following breaking changes:
- nvim_get_keymap now always returns a LuaRef object as "callback" for a
  Lua mapping regardless of how it is called. The LuaRef object can be
  called from Lua and Vim script, but is lost over RPC.
- maparg() now returns a Funcref instead of a ref number as "callback"
  for a Lua mapping. The Funcref can be called from Lua and Vim script,
  but is lost over RPC.

This may also make nvim_get_keymap faster, but make maparg() slower.

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

Problem:    The screen.c file is much too big.
Solution:   Split it in three parts. (Yegappan Lakshmanan, closes vim/vim#4943)
https://github.com/vim/vim/commit/7528d1f6b5422750eb778dfb550cfd0b0e540964

This is an approximation vim-patch 8.1.2057. Applying the patch directly
isn't feasible since our version of screen.c has diverged too much,
however we still introduce drawscreen.c and drawline.c:
- screen.c is now a much smaller file used for low level screen functions
- drawline.c contains everything needed for win_line()
- drawscreen.c contains everything needed for update_screen()

Co-authored-by: zeertzjq <zeertzjq@outlook.com>

Replace grid.h in screen.h and screen.h in buffer.h with grid_defs.h

The line returned but ml_get_buf() may be freed by another call to
ml_get_buf(), so it is necessary to make a copy.

use the MAXSIZE_TEMP_ARRAY + ADD_C pattern instead, as exemplified
by the changes in this commit.