problem: code for drawing statusline is arbitrarily spreadout between drawscreen.c, screen.c and buffer.c
solution: move it to a new file statusline.c

- rename archaic internal name "status match" to public name "wildmenu"
- showruler() does not show the ruler. it show anything which displays
  info about the cursor. Rename it accordingy.

Problem:    Get E685 and E931 if buffer reload is interrupted.
Solution:   Do not abort deleting a dummy buffer. (closes vim/vim#5361)
https://github.com/vim/vim/commit/a6e8f888e7fc31b8ab7233509254fb2e2fe4089f

This includes a partial port of Vim patch 8.2.2569 and some changes to
nvim_eval_statusline() to allow a multibyte fillchar. Literally every
line of C code touched by that patch has been refactored in Nvim, and
that patch contains some irrelevant foldcolumn tests I'm not sure how to
port (as Nvim's foldcolumn behavior has diverged from Vim's).

Problem:    Number of status line items is limited to 80.
Solution:   Dynamically allocate the arrays. (Rom Grk, closes vim/vim#7181)
https://github.com/vim/vim/commit/8133cc6bf454eb90bb0868f7cf806fce5c0c9fe6

The members of stl_item_T have not been prefixed with stl_ contrary to
the vim patch because the amount of stl_ prefixes on single lines of
code in that region was hurtful to readability.

Improve coverage of `build_stl_str_hl`.
Minor removal of dead code in the tested function.

Used

    sed -r -i -e '/ helpers =/ s/$/\nlocal itp = helpers.gen_itp(it)/; s/^(\s*)it\(/\1itp(/' test/unit/**/*_spec.lua

to alter all tests. Locally they all run fine now.

Reasoning:

1. General: state from one test should not affect other tests.
2. Local: travis build is failing with something which may be an output of
   garbage collector. This should prevent state of the garbage collector from
   interferring as well.

also allow handle==0 meaning curbuf/curwin/curtab

This new functionality is explained in the documentation.

Also, many tests have been added to the buffer_spec.lua file