Broken layer names when loading pickles from v0.6

[toumix]

From DisCoPy v0.6 run the following:

import pickle
from discopy import Ket, Measure
circuit = Ket(0) >> Measure(1)
f = open("circuit.pkl", "wb")
pickle.dump(circuit, f)
f.close()

From DisCoPy >1.0 we get a broken name for layers:

>>> import pickle
>>> f = open("circuit.pkl", "rb")
>>> print(pickle.load(f))
Layer >> Layer

This is because Layer.name is now calculated during the __init__.