Substantial refactor around PrefixMap

Extract a LightweightPrefixMap interface from the PrefixMap class (to avoid breaking existing code), plus extract some reusable methods from PrefixMap into a LightweightPrefixMapBase abstract class.

Add a FastPrefixMap implementation of the LightweightPrefixMap interface, this used a Trie (newly added to Atlas) to do fast abbreviation of URIs to prefixed name form.  This makes this implementation better suited to output uses, PrefixMap remains best suited for input since it uses less memory

Improves test coverage for prefix maps

git-svn-id: https://svn.apache.org/repos/asf/jena/trunk@1439363 13f79535-47bb-0310-9956-ffa450edef68