Make Loam Circuits Match Magma Circuits API

[David-Durst]

Currently, you can't get the string representation of some instances of loam boards. The magma repr function calls IO of the metaclass, and the loam board doesn't have an IO property.

>>> from loam.boards.icestick import IceStick
import lattice ice40
>>> icestick = IceStick()
>>> type(icestick)
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/durst/dev/W17-8/magma/magma/circuit.py", line 108, in __repr__
    args = str(cls.IO)
AttributeError: type object 'IceStick' has no attribute 'IO'
>>>