Users may add the expensive instances if they want.
The goal is to make sure the default configuration is solid, and
covers all examples we really want to support.
cc @kha @dselsam
The main issue is nontermination for problems that do not have
solution. When using dependent coercions, we keep creating goals of
the form `CoeSort ... (coe (coe (coe ...))) ...`. Same for `CoeFun`.
I am considering simplifying it even further, and making sure
`CoeDep` can be used at most once in a sequence of coercions `CoeTC`.
Another option is to use a very small amount of fuel to
guarantee termination when solving coercion TC problems.
cc @Kha @dselsam
