Use __slots__ for fast attribute access

lib/spack/llnl/util/lang.py

@@ -486,6 +486,8 @@ class HashableMap(MutableMapping):
     """This is a hashable, comparable dictionary.  Hash is performed on
     a tuple of the values in the dictionary."""
 
+    __slots__ = 'dict',
+
     def __init__(self):
         self.dict = {}