Current result of DecimalConversion benchmark (for future reference):

BenchmarkDecimalConversion-8	   10000	    204770 ns/op

Measured on Mac Mini (late 2012) running OS X 10.10.5,
2.3 GHz Intel Core i7, 8 GB 1333 MHz DDR3.

Also: Removed comment suggesting to implement decimal by representing
digits as numbers 0..9 rather than ASCII chars '0'..'9' to avoid
repeated +/-'0' operations. Tried and it appears (per above benchmark)
that the +/-'0' operations are neglibile but the addition conversion
passes around it are not and that it makes things significantly slower.

Change-Id: I6ee033b1172043248093cc5d02abff5fc54c2e7a
Reviewed-on: https://go-review.googlesource.com/14857Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Robert Griesemer <gri@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Alan Donovan <adonovan@google.com>