Refactor GF shell modules to improve modularity and reusability

+ Move type CommandInfo from GF.Command.Commands to a new module
  GF.Commands.CommandInfo and make it independent of the PGF type.
+ Make the module GF.Command.Interpreter independent of the PGF type and
  eliminate the import of GF.Command.Commands.
+ Move the implementation of the "help" command to its own module
  GF.Command.Help

src/compiler/GF/Command/CommandInfo.hs

@@ -0,0 +1,57 @@
+module GF.Command.CommandInfo where
+import GF.Command.Abstract(Option,Expr)
+import GF.Infra.SIO(SIO)
+import qualified PGF as H(showExpr)
+import qualified PGF.Internal as H(Literal(LStr),Expr(ELit)) ----
+import GF.Text.Pretty(Doc)
+
+data CommandInfo env = CommandInfo {
+  exec     :: env -> [Option] -> [Expr] -> SIO CommandOutput,
+  synopsis :: String,
+  syntax   :: String,
+  explanation :: String,
+  longname :: String,
+  options  :: [(String,String)],
+  flags    :: [(String,String)],
+  examples :: [(String,String)],
+  needsTypeCheck :: Bool
+  }
+
+emptyCommandInfo :: CommandInfo env
+emptyCommandInfo = CommandInfo {
+  exec = \_ _ ts -> return $ pipeExprs ts, ----
+  synopsis = "",
+  syntax = "",
+  explanation = "",
+  longname = "",
+  options = [],
+  flags = [],
+  examples = [],
+  needsTypeCheck = True
+  }
+--------------------------------------------------------------------------------
+
+class TypeCheckArg env where typeCheckArg :: env -> Expr -> Either Doc Expr
+
+--------------------------------------------------------------------------------
+
+newtype CommandOutput  = Piped {fromPipe :: ([Expr],String)} ---- errors, etc
+
+-- ** Converting command output
+fromStrings ss         = Piped (map stringAsExpr ss, unlines ss)
+fromExprs   es         = Piped (es,unlines (map (H.showExpr []) es))
+fromString  s          = Piped ([stringAsExpr s], s)
+pipeWithMessage es msg = Piped (es,msg)
+pipeMessage msg        = Piped ([],msg)
+pipeExprs   es         = Piped (es,[]) -- only used in emptyCommandInfo
+void                   = Piped ([],"")
+
+stringAsExpr = H.ELit . H.LStr -- should be a pattern macro
+
+-- ** Converting command input
+toString  = unwords . toStrings
+toStrings = map showAsString
+  where
+    showAsString t = case t of
+      H.ELit (H.LStr s) -> s
+      _ -> "\n" ++ H.showExpr [] t ---newline needed in other cases than the first