rework (make get/set_path generic Peekable, associated fns for metadata and next_path)

[jordens]

• a small step towards #104
• Changes the path assembly logic to postfix the separator. This is a bit easier in the logic.
• makes metadata() an associated fn
• homogenizes the API and e.g. makes the mqtt client and the unittests use set() where appropriate
• also makes the metadata semantically correct (depth N if there are N elements in the path)
• make next_path and related API associated fns
• Move the iterator state into the iterator
• fixes #88 by iterating all paths regardless of Option nulls.
• adds Error::PathAbsent to mark runtime absence in the case of Option::None.

[jordens]

@ryan-summers this rework PR escalated a bit. I'll stop before doing more significant changes.