A way to enforce uniqueness in typeclass instances

Most linear data structures require a way to enforce and maintain their uniqueness. And we have a few different ways to satisfy this requirement (see #130). However, some typeclass functions does not provide a way to enforce this property, so we can not implement them neatly as we'd like to.

Below is an example (originally posted at https://github.com/tweag/linear-base/issues/130#issuecomment-689509565)

Most operations on linear mutable vectors rely on the vector being unique, so the usual way to create a vector is to take a linear callback (alloc :: ... -> (Vector a #-> Unrestricted b) #-> Unrestricted b).

Today, I was implementing the Monoid instance for linear mutable vectors. Implementing mappend went well, since we already get two vectors, we can create a new one using the fact that we have access to some unique values.

However, I had a problem when implementing mempty :: a, because it doesn't provide a way to create a unique value. With the suggestion on #130 , we could have something like mempty :: Source #-> a instead. Otherwise, I need something like mempty :: forall b. (a #-> Unrestricted b) #-> b which is not pretty. Or we somehow have to accept that linear mutable vectors (at least with the current representation) are not monoids.

Start by reading this issue alongside #130, then examine the typeclass signatures discussed here, especially Monoid.mempty and the linear mutable vector example. Done would require an agreed design for enforcing uniqueness in typeclass instances; this issue names no files or tests.