feat(accounts): Decide on address creation scheme

[testinginprod]

We need to decide an address creation scheme for accounts created using x/accounts.

Currently the address package (not core.address) cannot be imported by x/accounts or it would leak the sdk dependency.

[robert-zaremba]

It should use the address package. If there is a dependency graph problem , then we can copy a function or move address to some other module.

[testinginprod]

yes, I think we'll want the address scheme to be: address.Module("accounts", append(creator_address, BigEndian(accNum)), this should avoid the risk of someone, besides the creator, front-running the account creation, since the adversary will not be able to produce the same address.

[tac0turtle]

@testinginprod is this closable?