Set DOTENV_LOOK_UPWARDS=1 by default

Fixes https://github.com/Tarrasch/zsh-autoenv/issues/3
Set DOTENV_LOOK_UPWARDS=1 by default

Change `_dotenv_hash_pair`, adding a version suffix to trigger
re-authentication.

Fixes https://github.com/Tarrasch/zsh-autoenv/issues/3
tests: add setup script, used to be for all tests
tests: inject '-t 1' to `read` during tests
Add varstash from smartcd

Source: https://github.com/cxreg/smartcd/blob/4fef3d3610873d9e8044cb5be1922fb056f4228d/lib/core/varstash
Adopt varstash from http://github.com/cxreg/smartcd

Decouple it from smartcd and consider it zsh-only.

This also adds required library functions from smartcd's
lib/core/arrays.
varstash integration

Fixes: https://github.com/Tarrasch/zsh-autoenv/issues/5
Comments / doc
s/ENV_AUTHORIZATION_FILE/AUTOENV_ENV_FILENAME/

Ref: https://github.com/Tarrasch/zsh-autoenv/issues/6
tests: add and use test_autoenv_add_to_env and test_autoenv_auth_env_files

Conflicts:
	autoenv.zsh
Streamline cwd behavior while sourcing
Cleanup
Use AUTOENV prefix

 - s/DOTENV_/AUTOENV_/
 - s/dotenv_/autoenv_/

Ref: https://github.com/Tarrasch/zsh-autoenv/issues/6
Some more refactoring, adding support for `autoenv_source_parent`

 - use file mtime with "entered" stack
 - fix loading of varstash; look at $functions
 - Refactor stack handling, add tests
 - Refactor `_autoenv_get_file_upwards`, add tests
 - tests: setup: reset AUTOENV_ENV_FILENAME