WIP/FYI: Subset constraints

[acl-cqc]

Around 20 tests failing; some probably need updating, but there are probably still some bugs yet. (Also some internal/unit tests of the inference algorithm that don't compile now, which I've commented out.) However I hope this includes basically the new code that we need, with only "tidying" and bugfixing still to come.

And, of course, there is Lukas' suggestion, maybe we don't need input_extensions (or an algorithm to compute them) at all...

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[acl-cqc]

Think we are going for #1142