Rename `bytes_to_int` to `bytes_to_uint64`
Use `encode_bytes` to implement `int_to_bytes`
Rename `int_to_bytes` to `uint_to_bytes` and move it to `ssz_impl.py`
1. Make sure that BLS -Verify APIs would only return `True` or `False` , no exceptions.
2. Use `eth2spec.utils.bls` instead of py_ecc for test generator
3. Add assertions in test generator
4. Add some special test cases for the -Verify APIs
5. Clean up the test format documents
Hsiao-Wei Wang
protolambda
Danny Ryan