Add FQP.copy()

[carver]

For a follow-up PR: I think this could be made a tiny bit more readable by adding a copy() method that takes the new coefficients, turning the line into:

return self.copy([x + y for x, y in  zip(self.coeffs, other.coeffs)])

From the current:

return type(self)([x + y for x, y in  zip(self.coeffs, other.coeffs)], self.curve_name)

Originally posted by @carver in https://github.com/ethereum/py_ecc/pull/41