avoid toolz in optimize.py

Fixes https://gitter.im/blaze/dask?at=5629d717d73f9e4f3b1b9202
As reported by @bnaul

dask/optimize.py

@@ -3,14 +3,16 @@
 from itertools import count
 from operator import getitem
 
-from toolz import identity
-
 from .compatibility import zip_longest
 from .core import (istask, get_dependencies, subs, toposort, flatten,
                    reverse_dict, add, inc, ishashable, preorder_traversal)
 from .rewrite import END
 
 
+def identity(x):
+    return x
+
+
 def cull(dsk, keys):
     """ Return new dask with only the tasks required to calculate keys.
 

dask/rewrite.py

@@ -1,6 +1,7 @@
 from __future__ import absolute_import, division, print_function
 
 from collections import deque
+
 from dask.core import istask, subs
 
 

dask/tests/test_optimize.py

@@ -1,9 +1,10 @@
 from itertools import count
 from operator import add, mul, getitem
-from toolz import partial, identity
+from functools import partial
 from dask.utils import raises
 from dask.optimize import (cull, fuse, inline, inline_functions, functions_of,
-        dealias, equivalent, sync_keys, merge_sync, fuse_getitem, fuse_selections)
+        dealias, equivalent, sync_keys, merge_sync, fuse_getitem,
+        fuse_selections, identity)
 
 
 def inc(x):