Factor out the Aux/AuxInt handling in (*Value).LongString() and
use it in (*Value).LongHTML() as well.
This especially improves readability of auxFloat32, auxFloat64,
and auxSymValAndOff values which would otherwise be printed as
opaque integers.
This change also makes LongString() slightly less verbose by
eliding offsets that are zero (as is very often the case).

Additionally, ensure the HTML is interpreted as UTF-8 so that
non-ASCII characters (especially the "middle dots" in some symbols)
show up correctly.

Change-Id: Ie26221df876faa056d322b3e423af63f33cd109d
Reviewed-on: https://go-review.googlesource.com/22641Reviewed-by: Josh Bleecher Snyder <josharian@gmail.com>
Run-TryBot: Josh Bleecher Snyder <josharian@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Frits van Bommel <fvbommel@gmail.com>