_dotenv_source: use 'builtin cd' and stored dir instead of 'cd -'
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 (experimental)