Refactor Source trait hierarchy

Refactor Source trait hierarchy

Source implements get/prefix/range, and is a subtrait of
IterSource which implements only iter(). The Default
implementation is now another subtrait of IterSource.

This avoids several of the previouas `Source<'a> + Ranges<'a>`
and 'Source<'a> + DefaultRanges' type constraints, and
simplifies type paths.