standaone cennznet utils

[jordy25519]

need a small package for cennznet/evm development.
it should be split from here so it doesn't have the complexity of the @polkadot packages (i.e. usable with current ethereum dapps without any changes)

functions needed:

• evm to cennznet address conversion
• generic asset id to address
• nft id to address

derive NFT address

AAAAAAAA + bytes(collectionId) + bytes(seriesId) + padRight(0, 4)

derive GA address

CCCCCCCC + bytes(assetId) + padRight(0, 12)
e.g. 16000 = 0xCCCCCCCC00003e80000000000000000000000000

cc #411