Occur-check in refine.

The occur check is done even if the flag [unsafe] is set to true. The rational is that a tactic cannot control where it takes pieces of terms from (and hence will not generally make terms which pass the occur-check), and it would be painful to ask every tactic which takes a term as an argument to do an occur check before [refine].

I reused the same error than used by unification. It gives a pretty nice error message. An alternative would be to have a dedicated error with pretty much the same error message. I'm not sure which is best, so I went for the simplest one.

The same check is done in the compatibility layer.

Fixes a reported bug which I cannot locate for some reason.

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