The compiler recognizes that in a sequence q = x/y; r = x%y only
one division is required. Remove prior work-arounds and write
more readable straight-line code (this also results in fewer
instructions, though it doesn't appear to affect the benchmarks
significantly).

name          old time/op  new time/op  delta
FormatInt-8   2.95µs ± 1%  2.92µs ± 5%   ~     (p=0.952 n=5+5)
AppendInt-8   1.91µs ± 1%  1.89µs ± 2%   ~     (p=0.421 n=5+5)
FormatUint-8   795ns ± 2%   782ns ± 4%   ~     (p=0.444 n=5+5)
AppendUint-8   557ns ± 1%   557ns ± 2%   ~     (p=0.548 n=5+5)

https://perf.golang.org/search?q=upload:20170310.1

Also:
- use uint instead of uintptr where we want to guarantee single-
  register operations
- remove some unnecessary conversions (before indexing)
- add more comments and fix some comments

Change-Id: I04858dc2d798a6495879d9c7cfec2fdc2957b704
Reviewed-on: https://go-review.googlesource.com/38071Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>