[m-rev.] for review: unexported types/insts/modes in interfaces

[Zoltan Somogyi]

This diff fixes Mantis bug 17, but as the diff shows, the library,
browser and compiler all had instances of this bug, and thus
it is very likely that other code does as well. Although referring
to locally-defined but unexported types, insts and modes was
documented to be non-allowed, the bug allowed them in practice,
so fixing this bug effectively makes till-now-accepted programs
no longer accepted by the compiler.

For review by anyone.

Zoltan.
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