Implement allowance functions

[Nemenwa]

Implement approve, allowance, and decrease_allowance functions following the SEP-41 token standard.
Function signatures:
rustpub fn approve(
env: Env,
from: Address,
spender: Address,
amount: i128,
expiration_ledger: u32,
) -> Result<(), ContractError>

pub fn allowance(env: Env, from: Address, spender: Address) -> i128

pub fn decrease_allowance(
env: Env,
from: Address,
spender: Address,
amount: i128,
) -> Result<(), ContractError>
Requirements:

approve must call from.require_auth()
Expired allowances must return 0 from allowance
expiration_ledger must be in the future
decrease_allowance should set to 0 if decrease exceeds current allowance (do not underflow)