Phase out use of VotingPower struct

[brentstone]

The VotingPower struct is just a wrapper around a u64, and with the decision to adopt votes_per_token = 1 per namnam, there isn't much use for passing around VotingPower in the code. If anything, this may still be used only in direct communication with tendermint, but otherwise should be replaced with an Amount or just a u64 itself. This will result in cleaner and more succinct code.