Added library for Unsigned FixedPoint calculations. UFixedPoint supports basic mathematical operations.

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_seconds_per_total_staked: UFixedPoint
}

and in storage

staking_log: Vec<StakingLogRecord>

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

Closes: #61