[New module system] Implement backtick imports

[yav]

This ticket is about imports of the form import `B which is a special form of functor instantiation (here B is a functor). Instead of taking a module implementing the arguments for the functor, we make an instance by adding the functor parameters as explicit parameters to each declaration.

This only succeeds if the parameter are simple---for example, since we do do not support higher rank types, the parameters of a module cannot be polymorphic.

This is a feature of the current module system, that's been useful to users, so the new module system should support it also.