| 0. `substEqInDict` needs to be symmetric (i.e., also apply right-to-left rules); try to re-use existing infrastructure. It would be neater, easier to understand, and more efficient to have one loop that goes for a fixed point of simultaneously rewriting with given_eqs, wanted_eqs, and type instances.
| 0. skolemOccurs for wanteds? At least `F a ~ [G (F a)]` and similar currently result in an occurs check error. Without skolemOccurs in wanted, the occurs check for wanted would need to be smarter (and just prevent cyclic substitutions of the outlined form silently). However, when inferring a type, having the rewrites enabled by skolemOccurs available will leads to potentially simpler contexts.
| 0. Comments:
| * When we raise a mismatch error in `TcSimplify` for unresolvable equalities, we effectively tidy the two non-matching types twice. Add a comment to highlight this and say way it is ok (i.e., they are never grouped together with `groupErrs` or similar).
| 0. `:t` in ghci doesn't print equalities in contexts properly.
| 0. ghci command to print normalised type and add [http://article.gmane.org/gmane.comp.lang.haskell.cafe/28799] as a test to the testsuite.