Make PGF.Tree internal

The only use of PGF.Tree outside the PGF library was in GF.Command.Commands,
and it was eliminated by using PGF.Expr directly instead.
PGF.Paraphrase still uses PGF.Tree.

gf.cabal

@@ -89,7 +89,6 @@ Library
     PGF.Macros
     PGF.Optimize
     PGF.Printer
-    PGF.Tree
     PGF.Utilities
   other-modules:
     PGF.CId
@@ -102,6 +101,7 @@ Library
     PGF.Parse
     PGF.Probabilistic
     PGF.SortTop
+    PGF.Tree
     PGF.Type
     PGF.Tokenizer
     PGF.TypeCheck