`reflection-examples`: Remove `newtype-generics` dependency

`newtype-generics` is [no longer actively maintained](https://github.com/sjakobi/newtype-generics/issues/18). Currently, `reflection-examples.cabal` depends on `newtype-generics`, so we should replace it with something else or remove it entirely.

Start with reflection-examples.cabal and search the repository for uses of newtype-generics. Determine whether the dependency can be removed or needs a replacement, then verify that the reflection-examples package still builds and works without the unmaintained dependency.

Written by the indexing model from the issue text.