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>

Problem:    Expand('<stack>') does not include the final line number.
Solution:   Add the line nuber. (closes vim/vim#6927)
https://github.com/vim/vim/commit/4f25b1aba050b85fa97ca2316aa04dd4b0b22530

Problem:    When a test fails it's often not easy to see what the call stack
            is.
Solution:   Add more entries from the call stack in the exception message.
https://github.com/vim/vim/commit/a5d0423fa16f18b4576a2a07e50034e489587a7d

Use docs from latest Vim.

Problem:    Execution stack is incomplete and inefficient.
Solution:   Introduce a proper execution stack and use it instead of
            sourcing_name/sourcing_lnum.  Create a string only when used.
https://github.com/vim/vim/commit/1a47ae32cdc19b0fd5a82e19fe5fddf45db1a506

Omit test_debugger.vim: superseded by later patches.
Omit check_map_keycodes(): N/A.
Omit kword_test.c: N/A (converted to a unit test).

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

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

The size of long varies depending on architecture, in contrast to the
MAXLNUM constant which sets the maximum allowable number of lines to
2^32-1. This discrepancy may lead to hard to detect bugs, for example
https://github.com/neovim/neovim/issues/18454. Setting linenr_T to a
fix maximum size of 2^32-1 will prevent this type of errors in the
future.

Also change the variables `amount` and `amount_after` to be linenr_T
since they're referring to "the line number difference" between two
texts.

related vim-8.2.{4402,4639}

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