compiler: refactoring in Export class to encapsulate type refs map

compiler: refactoring in Export class to encapsulate type refs map

    Convert the Export::type_refs map from a static object to a field
    contained (indirectly, via an impl class) in Export itself, for better
    encapsulation and to be able to reclaim its memory when exporting is
    done. No change in compiler functionality.

    Reviewed-on: https://go-review.googlesource.com/c/gofrontend/+/184170

From-SVN: r272919