Added a default slot of 1 to all turtle commands with slot parameters.
Removed unused file.
Added ability to provide more than one domToMutation() and mutationToDom()
method, which is required for a subsequent change to turtle_place.
Fixed bug with expression/statement blocks.
Minor cleanup, including removing unnecessary thisBlock assignments.
Fixed comment.
Folded turtle_place_sign into turtle_place.  This was tricky, since that
necessitated multiple mutationToDom() and domToMutation() methods for a
single block.