Also:
- make representation more flexible (no need to store trailing 0 digits to match precision)
- simplify rounding as a consequence
- minor related fixes

TBR adonovan

Change-Id: Ie91075990688b506d28371ec3b633b8267397ebb
Reviewed-on: https://go-review.googlesource.com/4841Reviewed-by: Rob Pike <r@golang.org>