Update to IETF BLS draft 04 and add some input validations

[hwwhww]

What was wrong?

Update to IETF BLS draft 04

How was it fixed?

1. SK=0 (point at infinity PK) is disallowed in -04 spec. Update KeyGen, KeyValidate correspondingly.
2. Add the missing KeyValidate to _CoreAggregateVerify: It wasn't a bug because _CoreAggregateVerify itself has checked pubkey_to_G1, but now it also needs to check the point at infinity PK.
3. Add input validations APIs _is_valid_privkey, _is_valid_pubkey, _is_valid_message, and _is_valid_signature. Reasons:
   • To check input types strictly since Python is a dynamically-typed language.
   • To follow the spec description more tightly.
   • As a workaround for PopVerify-able checks. e.g., ensure PK is not the point at infinity in FastAggregateVerify.

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[CarlBeek]

Aside from the validity of the point at infinity in the aggregate, this looks good to me. (To be clear, I both allowing it and disallowing it are fine with me as my reading of the specs has the behavior of Aggregate with the point at infinity as undefined.)