Template Haskell: allow type splices
At last! Trac #1476 and #3177 This patch extends Template Haskell by allowing splices in types. For example f :: Int -> $(burble 3) A type splice should work anywhere a type is expected. This feature has been long requested, and quite a while ago I'd re-engineered the type checker to make it easier, but had never got around to finishing the job. With luck, this does it. There's a ToDo in the HsSpliceTy case of RnTypes.rnHsType, where I am not dealing properly with the used variables; but that's awaiting the refactoring of the way we report unused names.