utils.gen: set __slots__.

We create *a lot* of these structs, so it saves a considerable
amount of space.

src/ircdb.py

@@ -688,7 +688,7 @@ class DuplicateHostmask(ValueError):
 class UsersDictionary(utils.IterableMap):
     """A simple serialized-to-file User Database."""
     __slots__ = ('noFlush', 'filename', 'users', '_nameCache',
-            '_hostmaskCache')
+            '_hostmaskCache', 'nextId')
     def __init__(self):
         self.noFlush = False
         self.filename = None

src/utils/gen.py

@@ -219,6 +219,7 @@ def exnToString(e):
 class IterableMap(object):
     """Define .items() in a class and subclass this to get the other iters.
     """
+    __slots__ = ()
     def items(self):
         if minisix.PY3 and hasattr(self, 'iteritems'):
             # For old plugins
@@ -252,6 +253,7 @@ class IterableMap(object):
 
 
 class InsensitivePreservingDict(collections.abc.MutableMapping):
+    __slots__ = ('data',)
     def key(self, s):
         """Override this if you wish."""
         if s is not None:
@@ -307,6 +309,7 @@ class InsensitivePreservingDict(collections.abc.MutableMapping):
 
 
 class NormalizingSet(set):
+    __slots__ = ()
     def __init__(self, iterable=()):
         iterable = list(map(self.normalize, iterable))
         super(NormalizingSet, self).__init__(iterable)