Make types.StrictFlags more strict

[lcwangchao]

types.StrictFlags is not "strict" in the strict sense beacuse of flag FlagClipNegativeToZero

The flag FlagClipNegativeToZero is zero in types.StrictFlags. It means when converting an negative to unsigned, the cast will succeed without any error or warning.

For example, if we converting -1 to unsigned. The types.StrictFlags will cast it to the max int value. Only if we set FlagClipNegativeToZero to 1, it will be recognized as an overflow error.

It's better to remove FlagClipNegativeToZero and introduce a new flag FlagAllowNegativeToUnsigned to replace it. When FlagAllowNegativeToUnsigned is not set by default, converting a negative value to unsigned will return an overflow error, otherwise, the negative value will be casted to an relatives positive value