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>

- 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

Problem:    src/ex_cmds.c file is too big.
Solution:   Move help related code to src/help.c. (Yegappan Lakshmanan,
            closes vim/vim#6506)
https://github.com/vim/vim/commit/f868ba89039045b25efe83d12ca501d657e170e8

Problem:    Mode not updated after CTRL-O CTRL-C in Insert mode.
Solution:   Set redraw_mode and use it. (closes vim/vim#10581)
https://github.com/vim/vim/commit/7a1d32809bb5c1527314000983e75125d79192e0

Problem:    The spell.c file is too big.
Solution:   Move the code for spell suggestions to a separate file. (Yegappan
            Lakshmanan, closes vim/vim#4988)
https://github.com/vim/vim/commit/46a426c9acfdd3d6c0fa134a17681634b9325bee

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

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

Problem:    Profiling functionality is spread out.
Solution:   Put profiling functionality in profiling.c. (Yegappan Lakshmanan,
            closes vim/vim#4666)
https://github.com/vim/vim/commit/fa55cfc69d2b14761e2a8bd85bc1e0d82df770aa

Move proftime_T to types.h for now to avoid recursive #include.