Stop retaining GRE closures

GRE closures should never be necessary to Haddock, so we never want to
keep them on the heap. Despite that, they are retained by a lot of the
data structures that Haddock makes use of.

- Attempt to fix that situation by adding strictness to various
  computations and pruning the `ifaceInstances` field of `Interface` to
  a much thinner data type.

- Removes the `ifaceFamInstances` field, as it was never used.

- Move some of the attach instances types (e.g. `SimpleType`) to the
  types module