fmt: optimize string truncation

fmt: optimize string truncation

Count only the runes up to the requested precision
to decide where to truncate a string.

Change the loop within truncate to need fewer jumps.

name                     old time/op  new time/op  delta
SprintfTruncateString-2   188ns ± 3%   155ns ± 3%  -17.43%  (p=0.000 n=20+20)

Change-Id: I17ca9fc0bb8bf7648599df48e4785251bbc31e99
Reviewed-on: https://go-review.googlesource.com/20098
Reviewed-by: Rob Pike <r@golang.org>