Add strict deserialization format validation

[hwwhww]

What was wrong?

Fix #108

How was it fixed?

Note: py_ecc doesn't provide APIs decompressed form deserialization, only compressed form deserialization.

1. Check if c_flag is True
2. If b_flag is True, check if a_flag is False and the value is 0.
3. For decompress_G2, check c_flag2 == b_flag2 == a_flag2 == False

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[hwwhww]

Thanks for the reviews! :)