Add support for literals

testsuite/lpgf/unittests/Literals.gf

@@ -4,4 +4,6 @@ abstract Literals = {
     mkString : String -> S ;
     mkInt : Int -> S ;
     mkFloat : Float -> S ;
+
+    and : S -> S -> S ;
 }

testsuite/lpgf/unittests/Literals.treebank

@@ -1,5 +1,5 @@
 Literals: mkString "hello"
-LiteralsCnc: hello
+LiteralsCnc: « hello »
 
 Literals: mkInt 123
 LiteralsCnc: 123
@@ -7,3 +7,6 @@ LiteralsCnc: 123
 Literals: mkFloat 30.809000
 LiteralsCnc: 30.809000
 
+Literals: and (mkString "one") (mkString "two")
+LiteralsCnc: « one » and « two »
+

testsuite/lpgf/unittests/Literals.trees

@@ -1,3 +1,4 @@
 mkString "hello"
 mkInt 123
 mkFloat 30.809
+and (mkString "one") (mkString "two")

testsuite/lpgf/unittests/LiteralsCnc.gf

@@ -1,7 +1,9 @@
 concrete LiteralsCnc of Literals = open Prelude in {
   lincat S = SS ;
   lin
-    mkString s = s ;
+    mkString s = ss ("«" ++ s.s ++ "»") ;
     mkInt s = s ;
     mkFloat s = s ;
+
+    and s1 s2 = ss (s1.s ++ "and" ++ s2.s) ;
 }