Serialization Fixes
* Normalize currencies made from 40 character hex strings into 3 character strings
* Parse Hash160 from `r.*` account id base58
* Ensure mantissa encoded into serialized amounts is absolute
* Make Amount.isNegative() public
* Parse hex strings with len % 2 of 1 for UINT
* Support legacy IOUs with 'XRP' currency
* Test serializing negative IOUs
* Test PathSet serialization