Turtle pretty-print: Blank node in object position of a single Statement

[ansell]

Where possible when pretty-printing Turtle, if a blank node is used as the object of a single Statement, and the subject of one or more other statements, the subject-use Statements should be nested under the single object Statement, to remove the use of a labelled blank node and replace it with an anonymous blank node notation.

[ansell]

This is a follow-up to the initial pretty-printing implementation added in issue #207