Add default ToSample implementation for Generics
Add default ToSample implementation for Generics

- introduce an internal `GToSample` class
- introduce internal functions `defaultSample` and `defaultSamples`
- add default signature for `toSamples` to use Generics
- set default `toSamples` implementation to `defaultSamples`
- remove the `MINIMAL` pragma to avoid warnings for empty instances

zerobuzz/servant,zerobuzz/servant