Add le() and lt() functions to SafeUint256 library

[kaliberpoziomka]

🧐 Motivation Currently functions uint256_le() and uint256_lt() from standard uint256.cario library are insecure. There are no checks if provided arguments are valid Uint256 type numbers. This may allow passing Uint256 objects with low and high member fields greater than 2**128 and in consequence return a false result.

📝 Details Functions SafeUint256.le() and SafeUint256.lt() would check if comparing numbers are valid Uint256 type numbers with uint256_check() function. After these checks valid arguments would be passed to uint256_le() / uint256_lt() functions and result from comparison would be returned.

[martriay]

For reference: https://github.com/starkware-libs/cairo-lang/blob/master/src/starkware/cairo/common/uint256.cairo#L132-L137

func uint256_lt{range_check_ptr}(a : Uint256, b : Uint256) -> (res : felt):
    if a.high == b.high:
        return is_le(a.low + 1, b.low)
    end
    return is_le(a.high + 1, b.high)
end