Added a method to SequentialAdder that returns the whole Contents.

(And not just the Value.)
Took advantage of SequentialAdder.getContents to avoid duplicating repr logic.
Added a method to MoleculeBuilder to set a lazy optional repr.

This is useful when whether the repr is present should be determined
lazily alongside with the repr itself.
Used MoleculeBuilder.setReprLazyO where appropriate.