Remove SDocs from ErrCtxt and ErrInfo

Currently we have

type ErrCtxt = (Bool, TidyEnv -> TcM (TidyEnv, SDoc))

data ErrInfo = ErrInfo {
    errInfoContext :: !SDoc
  , errInfoSupplementary :: !SDoc
  }

mkErrInfo :: TidyEnv -> [ErrCtxt] -> TcM SDoc

And the errInfoContext in ErrInfo in TcRnDetailedDiagnostic usually gets populated by mkErrInfo.

Proposed change 1: Change the SDoc in ErrCtxt, in errInfoContext, and in mkErrInfo to an appropriate ADT.

Also, there are I believe only two places which add a non-empty errInfoSupplementary. We have tc_infer_id:

    fail_with_msg whatName nm pprov = do
      (import_errs, hints) <- get_suggestions whatName
      unit_state <- hsc_units <$> getTopEnv
      let
        -- TODO: unfortunate to have to convert to SDoc here.
        -- This should go away once we refactor ErrInfo.
        hint_msg = vcat $ map ppr hints
        import_err_msg = vcat $ map ppr import_errs
        info = ErrInfo { errInfoContext = pprov, errInfoSupplementary = import_err_msg $$ hint_msg }
      failWithTc $ TcRnMessageWithInfo unit_state (
              mkDetailedMessage info (TcRnIncorrectNameSpace nm False))

Proposed change 2: Add [GhcHint] and [ImportError] to TcRnIncorrectNameSpace.

The other place is mkErrorReport:

mkErrorReport :: TcLclEnv
              -> TcRnMessage
                  -- ^ The main payload of the message.
              -> Maybe SolverReportErrCtxt
                  -- ^ The context to add, after the main diagnostic
                  -- but before the supplementary information.
                  -- Nothing <=> don't add any context.
              -> [SolverReportSupplementary]
                  -- ^ Supplementary information, to be added at the end of the message.
              -> TcM (MsgEnvelope TcRnMessage)
mkErrorReport tcl_env msg mb_ctxt supplementary
  = do { mb_context <- traverse (\ ctxt -> mkErrInfo (cec_tidy ctxt) (tcl_ctxt tcl_env)) mb_ctxt
       ; unit_state <- hsc_units <$> getTopEnv
       ; hfdc <- getHoleFitDispConfig
       ; let
           err_info =
             ErrInfo
               (fromMaybe empty mb_context)
               (vcat $ map (pprSolverReportSupplementary hfdc) supplementary)
       ; let detailed_msg = mkDetailedMessage err_info msg
       ; mkTcRnMessage
           (RealSrcSpan (tcl_loc tcl_env) Strict.Nothing)
           (TcRnMessageWithInfo unit_state $ detailed_msg) }

Proposed change 3: audit the calls to mkErrorReport to separately store the supplementary information in the TcRnMessage instead.