Do not discard insoluble Derived constraints
This is preparing for a fix to Trac #9612. The idea is that insoluble constraints are nice solid errors that we should not discard before we have a chance to report them. So TcRnTypes.dropDerivedWC now keeps insoluble Derived constrains, and instead TcSimplify.solve_wanteds filters them out We get somewhat better error message for kind-equality failures too. A slight downside is that to avoid *duplicate* kind-equality failures when we float a kind-incompatible equality (e.g. alpha:* ~ Int#), I've disabled constraint-floating when there are insolubles. But that in turn makes a handful of error messages a little less informative; good examples are mc21, mc22, mc25. But I am re-jigging the constraint floating machinery in another branch, which will make this go back to the way it was before.