Problem:    Some code in options.c fits better elsewhere.
Solution:   Move functions from options.c to other files. (Yegappan
            Lakshmanan, closes vim/vim#4889)
https://github.com/vim/vim/commit/e677df8d93772a705f40a94f3c871aee78fe4d99

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>

This reverts commit c4c74c3883aa3122c0c877ca8dd7b26beb5cc4aa.

LibUV already gives an error for this, so this isn't needed.

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

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

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

Problem:    The evalfunc.c file is too big.
Solution:   Move getchar() and parse_queued_messages() to getchar.c.
https://github.com/vim/vim/commit/9c658c9eacbd97e2c071f652a0155f71db94c0f3

Omit parse_queued_messages(): Nvim does not have MESSAGE_QUEUE.

Problem:    Code for the argument list is spread out.
Solution:   Put argument list code in arglist.c. (Yegappan Lakshmanan,
            closes vim/vim#4819)
https://github.com/vim/vim/commit/4ad62155a1015751a6645aaecd94b02c94c8934b