[m-rev.] for post-commit review: speed up string.format for integers some more

[Zoltan Somogyi]

This diff speeds up formatting integers by another 12% or so,
taking the time on that benchmark down to

15.841u 0.007s 0:15.86 99.8%	0+0k 0+0io 0pf+0w
16.051u 0.004s 0:16.06 99.9%	0+0k 0+0io 0pf+0w

Does anyone have an objection to raising the limit mentioned
in the diff from 2^15-1 to 2^31-1, i.e. to assuming that
Mercury platforms have at least 32 bit integers?

Zoltan.
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