Problem:
Subdirectories like "visual", "insert", "normal" encourage people to
separate *related* tests for no good reason.  Typically the _mode_ is
not the relevant topic of a test (and when it is, _then_ create
an appropriate describe() or it()).

Solution:
- Delete the various `test/functional/<mode>/` subdirectories, move
  their tests to more meaningful topics.
- Rename `…/normal/` to `…/editor/`.
  - Move or merge `…/visual/*` and `…/insert/*` tests into here where
    appropriate.
- Rename `…/eval/` to `…/vimscript/`.
  - Move `…/viml/*` into here also.

* test(reorg): insert/* => editor/mode_insert_spec.lua
* test(reorg): cmdline/* => editor/mode_cmdline_spec.lua
* test(reorg): eval core tests => eval_spec.lua

* eval: fix assertion failure in garbage collection

fixes #12387, #12430

Lists with CopyID+1 linked only from previous_funccal may be removed in
the garbage collection. Therefore, the terms of the assertions are not
correct. This can be confirmed by the following (The l:x with CopyID+1 of
the first function call needs to be removed by garbage collection):

func! s:f()
  let l:x = [1]
  let g:x = l:
endfunc

for _ in range(2)
  call s:f()
endfor
call garbagecollect()
" press any key

* test: add test for #12387, #12430

The test.functional.helpers and test.unit.helpers modules now include
all of the public functions from test.helpers, so there is no need to
separately require('test.helpers').

Note: the test fails on non-Windows CI (Travis linux, Quickbuild bsd):
even on master before the env.c changes in this patch-series.
Maybe the unix part of printenv-test.c needs to be revisited.

Signed-off-by: Justin M. Keyes <justinkz@gmail.com>

It's reported that the Windows widechar variants do automatically
convert from the current codepage to UTF16, which is very helpful. So
the "widechar" impls are a good direction.  But libuv v1.12 does that
for us, so the next commit will use that instead.

ref #8398
ref #9267

_putenv_s variant was left over from 810d31a43001, should have been
removed in cd5b1315757e.

Regression test coverage for #6070.

Main points:

- Replace `char_u` with `char` in some cases.
- Remove `str[len] = NUL` hack in some cases when `str` may be considered
  `const`.