[WIP] Feat staker v2

[baitcode]

Added new method to staker interface:

fn get_staked_seconds_at(self: @TContractState, staker: ContractAddress, timestamp: u64) -> u128;

Allows to calculate staked amount. If address staked 100 tokens for 10 seconds, that would be 1000 staked seconds. Fractions of a second are rounded down. Only full seconds count. Added additional data structure:

#[derive(Drop, Serde, starknet::Store)]
struct StakingLogRecord {
    timestamp: u64,
    total_staked: u128,
    cumulative_staked_per_second: u128
}

and in storage

staking_log: Map<ContractAddress, Vec<StakingLogRecord>>

and population logic into stake and withdraw methods. All searches in those logs are binary.

Closes: #61