add #[must_use] to all pub functions

[marcoieni]

docs here.

Calling config_dir() and all the other public functions without reading the return values makes no sense (because they have no side effects) and is a user error for sure.

By adding this macro, the user will get a warning if it calls these functions without reading the return value.

Example here

[tesuji]

This's maybe helpful.
You're welcome to create a PR to fix this issue.