[Staking] FullDelegation.accumulated_rewards should be `Vec<Coin>`

[ethanfrey]

Replaces the issue in #513 (read there for analysis and explanation)

This needs to change:

cosmwasm/packages/std/src/query.rs

Line 159 in c9a5141

pub accumulated_rewards: Coin,

This is easy to do, but API breaking. Plan for next tagged release