Remove some dead code

* The following modules are no longer used and have been removed completely:

	GF.Compile.Compute.ConcreteLazy
	GF.Compile.Compute.ConcreteStrict
	GF.Compile.Refresh

* The STM monad has been commented out. It was only used in
  GF.Compile.SubExpOpt, where could be replaced with a plain State monad,
  since no error handling was needed. One of the functions was hardwired to
  the Err monad, but did in fact not use error handling, so it was turned
  into a pure function.

* The function errVal has been renamed to fromErr (since it is analogous to
  fromMaybe).

* Replaced 'fail' with 'raise' and 'return ()' with 'done' in a few places.

* Some additional old code that was already commented out has been removed.

src/compiler/GF/Interactive.hs

@@ -8,7 +8,7 @@ import GF.Command.Interpreter(CommandEnv(..),commands,mkCommandEnv,emptyCommandE
 import GF.Command.Commands(flags,options)
 import GF.Command.Abstract
 import GF.Command.Parse(readCommandLine,pCommand)
-import GF.Data.Operations (Err(..),chunks,err,raise)
+import GF.Data.Operations (Err(..),chunks,err,raise,done)
 import GF.Grammar hiding (Ident,isPrefixOf)
 import GF.Grammar.Analyse
 import GF.Grammar.Parser (runP, pExp)
@@ -83,7 +83,7 @@ mainServerGFI opts files =
 
 -- | Read end execute commands until it is time to quit
 loop :: Options -> GFEnv -> IO ()
-loop opts gfenv = maybe (return ()) (loop opts) =<< readAndExecute1 opts gfenv
+loop opts gfenv = maybe done (loop opts) =<< readAndExecute1 opts gfenv
 
 -- | Read and execute one command, returning Just an updated environment for
 -- | the next command, or Nothing when it is time to quit
@@ -363,7 +363,7 @@ importInEnv gfenv opts files
            pgf1 <- importGrammar pgf0 opts' files
            if (verbAtLeast opts Normal)
              then putStrLnFlush $ unwords $ "\nLanguages:" : map showCId (languages pgf1)
-             else return ()
+             else done
            return $ gfenv { commandenv = mkCommandEnv pgf1 }
 
 tryGetLine = do