[m-dev.] Abstract exported dummy types

[Michael Day]

> But I would still prefer if users could define their own abstract
> exported dummy types, by adding a pragma in interface files.
> 
> We currently have "where type_is_abstract_enum" to expose something
> about the representation of values of an abstract type while maintaing
> the abstraction barrier, and I think we'll want something for
> fixed-width integer types, too. Exposing the dumminess of a type seems
> like just another aspect of the same idea.

This sounds better than silently falling back to a different 
representation or aborting, I think.

Michael

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