New encoding

This release introduces a new encoding for the id strings produced this library. It's more secure and extensible than the old encoding.

To maintain backward compatibility, the KeyStore type now optionally implements a new Versioner interface that reports its version. If the result is 2 or later, encoding produces the new format and decoding requires the new format. If 1 or less, or if a KeyStore does not implement Versioner, the old format is used.