Overflow issue when converting integers near maxint

[richcarl]

10 * total + code - 0x30 needs to be 10 * total + (code - 0x30), otherwise this overflows the integer range when converting values close to maxint (2^53 - 1 = 9007199254740991 with a JS backend). For example:

    > String.toInt "9007199254740945"
    Just 9007199254740944 : Maybe Int
    > String.toInt "9007199254740991"
    Just 9007199254740992 : Maybe Int

https://github.com/elm/core/blob/84f38891468e8e153fc85a9b63bdafd81b24664e/src/Elm/Kernel/String.js#L291C24-L291C35

I would expect this function to always give an exact result whenever the integer literal fits into the available range for integers, but this is currently not true.

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