Allow Include What You Use to remove unnecessary includes and only
include what is necessary. This helps with reducing compilation times
and makes it easier to visualise which dependencies are actually
required.

Work on https://github.com/neovim/neovim/issues/549, but doesn't close
it since this only works fully for .c files and not headers.

Lua makes (or reuses) an internal copy of strings, so we can safely push
buf pointers onto the stack.

- 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>