Rewrite stash handling

There are no more grafts, no more real concept of "stash paths", and
Foo::Bar really does define and use package Foo now.  As per jnthn's
design, non-packages can now replace packages in place, allowing
class Foo::Bar; class Foo to work again.  Things are generally
simpler; starting support for CORE:: and friends.

TODO

@@ -104,4 +104,3 @@ Other stuff to do after:
   - jnthnian packages
   - pm's = hack
   - fix { my $x } crash
-  + checking redeclaration of our symbols, methods, attributes...

lib/Builtins.cs

@@ -1213,6 +1213,23 @@ class SubstrLValue: Variable {
         return File.Exists(path) || Directory.Exists(path);
     }
 
+    static string[] split_on_colons(string path) {
+        List<string> l = new List<string>();
+        int ix = -2;
+        do {
+            int nix = path.IndexOf("::", ix+2);
+            l.Add(nix < 0 ? path.Substring(ix+2) :
+                    path.Substring(ix+2, nix - (ix+2)));
+            ix = nix;
+        } while (ix >= 0);
+        return l.ToArray();
+    }
+
+    public static Variable dynamic_package_var(Frame th, string path) {
+        string[] toks = split_on_colons(path);
+        return null;
+    }
+
     public static Variable BoxLoS(string[] los) {
         VarDeque items = new VarDeque();
         foreach (string i in los)