chore: use keccak256 for blockhash

[minaminao]

As the discussion in foundry-rs/foundry#6235 , currently, blockhash(number) returns number, but I think we should use keccak256 for blockhash.

The main reason is:

blockhash, even though it's a 256-bit value, can be represented by a much smaller number of bits from a security point of view.
For example, if a contract has a bug that the upper bits of the blockhash value are missing in inline-assembly or something, the bug would not be detected in tests.

This PR used it as we have done so before #691 .

@DaniPopes This is a minor change and mds1 agreed that so I've sent this PR to facilitate it before I get your reply. :)

[rakita]

@DaniPopes waiting for your approval before merging