parse: honor target bit size for integer strings

[team-humaki]

parseInt/parseUint called strconv.ParseInt/ParseUint with bitSize 0, so a string that fits in a platform int but not the destination type wrapped and returned err == nil. Overflow past int64 already failed, so the string path disagreed with itself depending on width.

Pass the destination bit size. ToInt8E("128"), ToInt8E("-129"), and ToUint8E("256") now error; in-range values are unchanged.

Fixes #260

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}