This is simplified faster version with much more lower lines of code of
https://github.com/Tarrasch/zsh-autoenv.

Possibly harmful modifications:

* Removed debugging messages. Only the fact that autoenv file is sourced
* Removed autoenv_*_path helpers -- "path" array is already simple and
  convenient to work with
* Removed workaround function when zsh/stat module is lacked
* Removed autoenv_source_parent() -- useless for me
* Removed autoenv-edit() -- too trivial task
* No testing provided

Optimizations:

* Removed old checksum format version support
* Less subshell invocations -- only cksum is left
* Heavily simplified and faster _autoenv_get_file_upwards() -- original
  was compatible with an old zsh versions
* varstash library is loaded without checking if autoenv needs it -- it
  is faster to load it anyway, than to invoke grep every time
* ctime is used instead of mtime, without size storage (unnecessary)

Look USAGE for examples.