Conversions between lazy and strict StateT

Should there be functions to convert between strict and lazy `StateT` variants?

```haskell
strictToLazyStateT :: Strict.StateT s m a -> Lazy.StateT s m a
strictToLazyStateT (Strict.StateT f) = Lazy.StateT f

lazyToStrictStateT :: Lazy.StateT s m a -> Strict.StateT s m a
lazyToStrictStateT (Lazy.StateT f) = Strict.StateT f
```

The issue names no files or tests. Start by locating the strict and lazy StateT definitions in mtl and comparing their constructors and public APIs. Determine whether the two proposed conversions preserve the intended behavior, then seek a maintainer decision; done means an agreed API direction with tests if the functions are accepted.

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