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>

vim-patch:8.1.2082: some files have a weird name to fit in 8.3 characters

Problem:    Some files have a weird name to fit in 8.3 characters.
Solution:   Use a nicer names.
https://github.com/vim/vim/commit/30e8e73506e4522ef4aebf7d525c0e6ffe8805fd

- reimplement 'winhl' in terms of highlight namespaces
- check for EOF in screen tests (to indicate a likely crash)

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

Mostly avoids including eval.h, ex_cmds2.h and ex_docmd.h in other
headers.

This avoids including ex_eval.h in any other header, thus preventing
future circular includes.

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

Problem:    File for Insert mode is much too big.
Solution:   Split off the code for Insert completion. (Yegappan Lakshmanan,
            closes vim/vim#4044)
https://github.com/vim/vim/commit/7591bb39d58ece38a5fef984a08ea9012616c1f9

Cherry-pick ins_compl_len() -> get_compl_len() from patch 8.2.4001.

Revert a71c5e9eb98fbb2ca88510269935cdcda37369fc: ctrl_x_mode is no
longer a global variable, so l_ctrl_x_mode is no longer needed.