Measure voting power in units of staking token

[plaidfinch]

Per @hdevalence:

the voting power is measured in units of "stake that has been bonded from genesis with 0 commission and no slashing" ... we should change the math so that we measure voting power just in units of "staking token"

Currently, voting power is calculated as:

https://github.com/penumbra-zone/penumbra/blob/0f099676441885eecc0fcba7bc3b71e00c7c403d/component/src/stake/rate.rs#L131-L138

I think what we want is:

 pub fn voting_power(&self, total_delegation_tokens: u64) -> u64 { 
     total_delegation_tokens
        .checked_mul(self.validator_exchange_rate)
        .expect("voting power in possible range")
 }

In other words, removing the denominator of base_rate_data.base_exchange_rate means that we would no longer be scaling all voting power by the (global) base exchange rate for a "perfect validator" who has existed since genesis.

In the above, the effect of u64::checked_mul is the same as casting up to u128 and then checking on the downcast after the multiplication, since we don't have an intermediate division.

[hdevalence]

This looks good, and we should also adjust the formulas in the spec while we do it.

[redshiftzero]

Blocked on #1481

[plaidfinch]

Blocked on using U128x128 for formulae.