The trouble was that my changes made a lot more files transitively link with
DynFlags, which is the root module for the revolting Windows dll-split stuff.

Anyway this patch fixes it, in a good way:

 - Make GHC/Hooks *not* import DsMonad, because DsMonad imports too
   much other stuff (notably tcLookup variants).  Really, Hooks depends
   only on *types* not *code*.

 - To do this I need the DsM type, and the types it depends on,
   not to be part of DsMonad.  So I moved it to TcRnTypes, which is
   where the similar pieces for the TcM and IfM monads live.

 - We can then delete DsMonad.hs-boot

 - There are a bunch of knock-on change, of no great significance