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>

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

- Remove autocmd.h from fileio.h
- Remove normal.h from main.h
- Move bufinfo_T from undo_defs.h to undo.c

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.

vim-patch:8.2.0559: clearing a struct is verbose

Problem:    Clearing a struct is verbose.
Solution:   Define and use CLEAR_FIELD() and CLEAR_POINTER().
https://github.com/vim/vim/commit/a80faa8930ed5a554beeb2727762538873135e83

Problem:    Code has more indent than needed.
Solution:   Use continue and return statements. (closes vim/vim#10824)
https://github.com/vim/vim/commit/101d57b34b72f4fbc7df1b6edfd64c64a6be14fc

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

Work on https://github.com/neovim/neovim/issues/567