Add FQP.copy()

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

From the current:
```py
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_

Start at the FQP class and its coefficient addition implementation. Confirm how instances preserve their curve information, then add the requested copy() entry point and use it for the shown expression. Done means the addition path uses copy() with the new coefficients and the existing behavior remains unchanged.

Written by the indexing model from the issue text.