Problem:    Writing viminfo fails with a circular reference.
Solution:   Use copyID to detect the cycle. (closes vim/vim#6217)
https://github.com/vim/vim/commit/5b157fe2edfdce5f77080aeac2b4a03f39eb1c1a

Fix the problem of failing to write shada when the global variable contains
Funcref or Partial.

Fixes #6957

https://github.com/mpeterv/luacheck/pull/81#issuecomment-261099606

> If you really want to use bleeding-edge version you should get the
> rockspec from master branch, not a fixed commit ...
> The correct way to install from a specific commit is cloning that
> commit and running "luarocks make" from project directory. The reason
> is that running "install" or "build" on an scm rockspec fetches
> sources from master but uses build description from the rockspec
> itself, which may be outdated.

In order to not conflict with new error codes that Vim adds, all Neovim
error codes should be above 5000.  The three existing sub-5000 error
codes (E926, E951, and E952) are now E50003, E5004, and E5005
respectively.

E953 was removed in 6167ce6df2753d5474ad49aea19f5957128ab015, so just
remove it from the help.

It is otherwise impossible to determine which test failed sanitizer/valgrind
check. test/functional/helpers.lua module return was changed so that tests which
do not provide after_each function to get new check will automatically fail.

Closes #3721

Note: it looks like viminfo files do not store search direction intentionally.
After reading viminfo file search direction was considered to be “forward”.

Note 2: all files created on earlier Neovim version will automatically receive
“forward” direction.

Fixes #3580

- Remove unused variables.
- Do not use helpers.nvim_feed in most cases.
- Do not use helpers.nvim and helpers.nvim_eval at all.
- Add helpers.funcs and helpers.\*meths special tables. Indexing such table 
  creates functions which call helpers.call or helpers.nvim (and similar) with 
  first argument equal to table index.