Make CompositeDevice._named a frozendict (and add frozendict to compat.py)

This prevents it being modified post-construction (just like the way
CompositeDevice._all and CompositeDevice._order are already 'frozen' by being
tuples)

gpiozero/compat.py

@@ -9,6 +9,9 @@ from __future__ import (
 str = type('')
 
 import cmath
+import collections
+import operator
+import functools
 
 
 # Back-ported from python 3.5; see
@@ -51,3 +54,30 @@ def median(data):
         i = n // 2
         return (data[i - 1] + data[i]) / 2
 
+
+# Copied from the MIT-licensed https://github.com/slezica/python-frozendict
+class frozendict(collections.Mapping):
+    def __init__(self, *args, **kwargs):
+        self.__dict = dict(*args, **kwargs)
+        self.__hash = None
+
+    def __getitem__(self, key):
+        return self.__dict[key]
+
+    def copy(self, **add_or_replace):
+        return frozendict(self, **add_or_replace)
+
+    def __iter__(self):
+        return iter(self.__dict)
+
+    def __len__(self):
+        return len(self.__dict)
+
+    def __repr__(self):
+        return '<frozendict %s>' % repr(self.__dict)
+
+    def __hash__(self):
+        if self.__hash is None:
+            hashes = map(hash, self.items())
+            self.__hash = functools.reduce(operator.xor, hashes, 0)
+        return self.__hash