There were cases where the currency and integer were incorrectly matched. By separating out the regex for hex formatted Amount makes it easier to deal with these cases and fixes the issue.
ISO 4217 states support for both alphabetic and numeric codes. rippled adheres to the ISO 4217 as stated in the Currency_Format on the wiki (https://ripple.com/wiki/Currency_Format) and there are trustlines out there with numeric currency codes.
The three-digit numeric code is useful when currency codes need to be understood in countries that do not use Latin scripts and for computerised systems. Where possible the 3 digit numeric code is the same as the numeric country code.
Amount#parse_quality is made currency-aware. This allows it to adjust for XRP as
the base currency, as well as for interest-bearing or demurring base currencies.
This patch might regress the performance of the Currency class and by extension
the Amount class. Since Amount is on a lot of hot paths in the client we should
make sure this isn't a major problem.
As for compatibility, this patch is a major change, but it should maintain the
public interface very well, which the exception of some strange edge cases (e.g.
Currency.from_json(1337)), which weren't well-defined before anyway. Any code
that accesses _value directly (shame on you!) will need to be fixed. There
aren't any such references in ripple-client or the rippled test suite, so I
think we're looking pretty good.
