Implement balance and transfer functions

[Nemenwa]

Description:
Implement the core token transfer functions: balance, transfer, and transfer_from.
Function signatures:
rustpub fn balance(env: Env, id: Address) -> i128

pub fn transfer(
env: Env,
from: Address,
to: Address,
amount: i128,
) -> Result<(), ContractError>

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

transfer must call from.require_auth()
transfer_from must call spender.require_auth() and check allowance
Both transfer functions must call the compliance contract's check_transfer_allowed(from, to) before executing. If compliance check fails, return ContractError::TransferNotAllowed.
Transfer amount must be > 0. If not, return ContractError::InvalidAmount.
Sender must have sufficient balance. If not, return ContractError::InsufficientBalance.
On successful transfer, emit a Transfer event.

Acceptance criteria:

All three functions implemented
Compliance check integration tested (mock the compliance contract)
All error cases covered
Events emitted correctly