[EXPERIMENT] Fields as subclass of int

### What is wrong?

Currently, the `FQ` and `FQP` fields have a lot of `isinstance` checks. This can be eradicated by making the classes subclasses of `int`.

Reference Comment - https://github.com/ethereum/py_ecc/pull/41#discussion_r262129838

### How can it be fixed

Fill this in if you know how to fix it.

Start by locating the FQ and FQP class definitions and the isinstance checks they currently require. Read the linked pull-request discussion for the proposed direction; done means the classes can subclass int and the unnecessary checks are removed without breaking the existing curve operations.

Written by the indexing model from the issue text.