Add Iterator to allow iterating over all entries of a trie

[cowtowncoder]

It would be useful to allow iteration over entries of a trie -- TrieDumper already implements this for simple output, but this would have other uses as well, esp. converting to other data structs.

Given that internal storage is nothing like that for Maps, the primary interface should NOT be Map's iterator (that would give wrong ideas of optimal usage), but for compatibility it is possible to also provide access as Map.Entry instances (for JDK Maps this is optimal interface because entries returned are part of Map; for trie there is no such internal structure to use).