Use the SIO monad in the GF shell

+ The restrictions on arbitrary IO when GF is running in restricted mode is now
  enforced in the types.
+ This hopefully also solves an intermittent problem when accessing the GF
  shell through the web API provided by gf -server. This was visible in the
  Simple Translation Tool and probably caused by some low-level bug in the
  GHC IO libraries.

src/compiler/GF/Infra/UseIO.hs

@@ -197,16 +197,6 @@ writeUTF8File fpath content = do
 readBinaryFile path = hGetContents =<< openBinaryFile path ReadMode
 writeBinaryFile path s = withBinaryFile path WriteMode (flip hPutStr s)
 
--- * Functions to limit acesss to arbitrary IO and system commands
-restricted io =
-    either (const io) (const $ fail message) =<< try (getEnv "GF_RESTRICTED")
-  where
-    message =
-      "This operation is not allowed when GF is running in restricted mode."
-
-restrictedSystem = restricted . system
-
-
 -- Because GHC adds the confusing text "user error" for failures cased by
 -- calls to fail.
 ioErrorText e = if isUserError e