Handle 0X in String to integer conversions

[RX14]

Currently String#to_unsigned_info handles only 0x, but sometimes 0X appears too. Should this be supported?

[straight-shoota]

I suppose it makes sense to support this. Not sure if there would be a reason to make this configurable?
It should be fine to have case-insensitive prefix enabled by default, I think.

For reference, Python's int() supports this as well.

In any case, we should include the other base prefixes as well: 0X, 0O, 0B.