We use `MProd` instead of `Prod` to group values when expanding the
`do` notation. `MProd` is a universe monomorphic product.
The motivation is to generate simpler universe constraints in code
that was not written by the user but generated by the `do` macro.
Note that we are not really restricting the macro power since the
`HasBind.bind` combinator already forces values computed by monadic
actions to be in the same universe.
The new test cannot be compiled without this modication.
7bfa39ae45