Fix Int64.toRadixStringUnsigned extra leading digits

[osa1]

Currently Int64.toRadixStringUnsigned adds an extra '0' when the result is single digit, e.g. '02' instead of '2'.

To be able to use the standard library toRadixString, we split the number into two halves, then calculate the lowest digit and the rest of the digits separately. With the lowest digit removed, the rest of the digits can be passed as a positive integer to toRadixString. The resulting strings are then concatenated to get the final result.

The problem occurs when the number can be printed as a single digit. In this case the rest of the number is printed as '0', giving us e.g. '02' instead of '2'.

To fix, we return early by calling the standard library toRadixString when the number is positive.

This works because

• The problem is only when printing single digits (as the lowest digit and the rest are printed separately, and the problem is when the "rest" part is 0)

• Single-digit numbers need to have high 32-bits as zero (otherwise they'll be negative and printed as many digits, or positive but larger than single digit).

• When the high 32-bits are all zero, the number is positive, and can be converted to string directly with the standard library signed toRadixString.

[github-actions]

Package publishing

Package	Version	Status	Publish tag (post-merge)
package:args	2.8.0	ready to publish	args-v2.8.0
package:async	2.13.1-wip	WIP (no publish necessary)
package:characters	1.4.1	already published at pub.dev
package:collection	1.20.0-wip	WIP (no publish necessary)
package:convert	3.1.3-wip	WIP (no publish necessary)
package:crypto	3.0.7	already published at pub.dev
package:fixnum	1.2.0-wip	WIP (no publish necessary)
package:lints	6.1.0-wip	WIP (no publish necessary)
package:logging	1.3.1-wip	WIP (no publish necessary)
package:os_detect	2.0.4-wip	WIP (no publish necessary)
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package:platform	3.1.7-wip	WIP (no publish necessary)
package:typed_data	1.4.1-wip	WIP (no publish necessary)

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[github-actions]

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