Consider adding a type `ProductT`

I was looking for a type

```
data ProductT (f :: (* -> *) -> * -> *) (g :: (* -> *) -> * -> *) m a
= PairT (f m a) (g m a)
```

and was pleasantly surprised that this package already provides its sibling `ComposeT`.

Would a type `ProductT` be accepted in this package?

Start by locating the existing ComposeT type and reviewing how it is exposed by this package. Check the package's conventions before deciding whether ProductT belongs in the public API; done means the proposal receives a maintainer decision and, if accepted, ProductT is included consistently with ComposeT.

Written by the indexing model from the issue text.