Add relevant instances for Gen from zio-test

[nbenns]

It would be nice to be able to use prelude combinators when creating Generators using zio-test, for example IdentityBoth, Covariant instances to support mapN, forEach and flip.

However, adding instances to zio-prelude itself would cause it to depend on zio-test and therefore pull zio-test into application code.

Should there be separate sub-module created to support these test instances?
something like zio-prelude-test for example.

[adamgfraser]

I think ZIO Prelude Laws could be a logic place for this since it already depends on both ZIO Prelude and ZIO Test and provides other relevant functionality that requires both libraries (e.g. generators for ZIO Prelude data types, laws for ZIO Prelude data types).