[KIP-114] block number is serialized to 32 bytes

[blukat29]

Modify KIP-114 spec's implementation detail.

• Change block_number_to_bytes() from an 8-byte big-endian bytes to a 32-byte big-endian bytes.
• Add a test case to check implementation correctness.
• Change RANDOM opcode semantic to return the current block's, not latest block's.

Reason

• In Go implementation, passing message other than 32-bytes is inconvenient because Klaytn's bls.VerifySignature expects 32 bytes message (following the Prysm's convention)

[blukat29]

@kjhman21 @aidan-kwon This PR is a fix to match with implementation. PTAL.

[kjhman21]

I have no objection for the change, but one question:

Is this required another hard fork or does it have compatibility with the previous spec?

[blukat29]

@kjhman21 Klaytn v1.12.0 is already implemented in the new spec (this PR; 32-byte big-endian bytes) (https://github.com/klaytn/klaytn/blob/v1.12.0/consensus/istanbul/backend/randao_test.go#L20).

The specs before & after this PR are incompatible, but the previous spec has never been instantiated.