Loading some modules takes longer than expected

[ghost]

This is just a follow up to the conversation I had with @brianhuffman about certain modules (around 2K loc) taking longer than expected. I can provide further details on request.

[brianhuffman]

After looking into this, it seems that the @@ operation to combine Subst values (used for unification of type variables during type checking) becomes a bottleneck when the substitution gets large. I have some ideas for different ways to implement this more efficiently.

[brianhuffman]

Switching Subst to use IntMap instead of Map gives a significant constant-factor speedup. However, the asymptotic run-time performance is still bad: at least cubic in the size of the input.

[brianhuffman]

The problematic input mentioned in the original post was an unusually difficult case, in that it was not only big, but contained a very high number of local declarations inside where blocks. We expect that situations where the Subst values in unification are a performance bottleneck will be quite rare, so we'll close this ticket, at least until it becomes a problem for someone again.