Emit error for dangerous vars at top level.

[benl]

Exporting mutable data containing monomorphic type vars is not sound because two different modules might update it at different types.

At the moment we seem to silently default such types to Unit.

thing 	:: forall a. Ref %r1 (a -> a)
	:- Mutable %r1

thing	= Ref id

In the .di file we get

foreign import thing
        :: Base.Ref %r1 (Base.Unit -> Base.Unit)
        :- Base.Mutable %r1
        ,  Base.Direct %r1
        :$ Base.Data;

[benl]

Test is T50-TopLevelMonoMutable