* Allow env vars to be kept from removed overlay
* Rename --keep to --keep-custom; Add new test
* Rename some symbols
* (WIP) Start working on --keep for defs and aliases
* Fix decls/aliases not melting properly
* Use id instead of the whole cloned overlay
* Rewrite overlay remove for no reason
Doesn't fix the bug but at least looks better.
* Rename variable
* Fix adding overlay env vars
* Add more tests; Fmt + Clippy