Integer bounds error in Vyper

Vyper is a pythonic Smart Contract Language for the ethereum virtual machine. In affected versions, the return of `<iface>.returns_int128()` is not validated to fall within the bounds of `int128`. This issue can result in a misinterpretation of the integer value and lead to incorrect behavior. As of v0.3.0, `<iface>.returns_int128()` is validated in simple expressions, but not complex expressions. Users are advised to upgrade. There is no known workaround for this issue.