Print encodings as [R#10c] instead of [R/10c].

The # is a more conventional prefix for hexadecimal, and when ISA
information is not available, there may be a decimal number in front
which would be confusing.

So prefer [1#10c] for the ISA-less encoding format. Here '1' is decimal
and '#10c' is hexadecimal.

meta/cretonne/__init__.py

@@ -1205,7 +1205,7 @@ class Encoding(object):
         self.isap = And.combine(recipe.isap, isap)
 
     def __str__(self):
-        return '[{}/{:02x}]'.format(self.recipe, self.encbits)
+        return '[{}#{:02x}]'.format(self.recipe, self.encbits)
 
     def ctrl_typevar(self):
         """