Ensure hash(Key(…)) is independent of dim order

[khaeru]

With:

from genno import Computer, Key

# Two keys with same dimensions but different order
k1 = Key("foo", "abc")
k2 = Key("foo", "cba")

d = {k1:"bar"}
k1 in d  # True
k2 in d  # False

This is because:

hash(k1) != hash(k2)  # 5471437777101518998 != 2185684392273789077

Although the following do work:

c = Computer()
c.add(k1, None)

k2 in c  # True
k2 in c.graph  # True

To resolve: make Key.__hash__() independent of dimension order.