To add grouping to a number, we take a string such as "123456.123" and
break it into integer and fraction parts. Then we take the integer part
and break it into locale-specific sized groups to inject the locale's
group separator (e.g. a comma in en-US). We currently create new strings
for each of these groups. Instead, we can use the shared superstring
method to avoid all of that string copying.
b4113536ef