currency: refactored currency-related code.

Moved currency-related code from language to a separate package.
This will fit better in the grand design for localization support.

Also exposed rounding modes in new API.

Change-Id: Iad8e0988241b81312c4c4458db0ee5f16c71f6a9
Reviewed-on: https://go-review.googlesource.com/13855
Reviewed-by: Nigel Tao <nigeltao@golang.org>
8 files changed