Static blake2b and OperationListListHash & BlockPayloadHash helpers

[m-kus]

Hey! Here are two new changes:

• first is making blake2b static (to avoid instantiating Crypto every time), instance method is kept for backward compatibility
• second one is more specific, it is a port of generic merkle tree implementation used in Tezos protocol and its application for calculating root hashes in the block header (operation list list and block payload) — pls let me know if it is something you want to have in the SDK, or I can keep it outside

[RomarQ]

Thank you again for the contribution.

I will try to review it later today or tomorrow.

[RomarQ]

I have not forgotten about this PR. Just did not yet have the time to look into the specifics.
Hopefully will have a look next week.

[m-kus]

No worries :)
I think it requires some changes, similar to the coder: need to expose additional api rather than make internal mod public.
Will do tomorrow