[m-rev.] for review: allow higher-order inst info in equivalence types

[Zoltan Somogyi]

For review by anyone.

This diff starts the process of allowing higher-order inst info in types.
It specifically allows them on the right hand side of equivalence type
definitions, which is enough for an important use case. getopt_io.m,
which normally uses *lots* of insts to specify higher order inst info,
can now get *all* of its higher-order inst info purely from the types.
(The attached new version of getopt_io.m still uses four insts,
but it uses them for another purpose.)

The process that this diff starts has a long way to go. The type
why_no_ho_inst_info in parse_type_name.m lists many more places where
higher-order inst infos are syntactically possible but are also semantically
prohibited, at least for now. In some cases, the prohibition should probably
stay (I see no point in lifting it for ctgc/sharing/reuse pragmas, since
they are not yet operational); in others, such as pred and func args and
mutable decls, it should go. But even in the cases where we want to lift
the prohibition, we need to agree on what the updated semantics is
of the construct in question, and create test cases for the proper
implementation of that semantics.

The only practical way to do this is lifing each restriction piece-by-piece,
and so that is what I propose to do. If I find the semantics of lifting
a restriction unclear, I will describe the issues and wait for a consensus
before proceeding. Are there any objections?

I don't think there is any point in announcing each step to users. I think
any announcements should wait until the end, because only then will the
picture for users be simple again.

Zoltan.
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