Some panic!s could be represented as unimplemented!s

**Is your feature request related to a problem or challenge? Please describe what you are trying to do.**
No

**Describe the solution you'd like**
If something is currently unimplemented and may be in the future, it is more fluent in Rust to use `unimplemented!`

**Describe alternatives you've considered**
Leave the panic!

**Additional context**
N/A
