TreeAny, accessors/rename, JSONPath

[jordens]

From the changelog:

Changed

• [breaking] The Traversal error enum has been split from the Error<E> enum to reduce genericism.
• [breaking] Increment trait and blanket impl removed in favor of increment_result/
  Error::increment/Traversal::increment
• Uncounted iteration is the default
• [breaking] The traverse_by_key callback receives the field name as an Option<&'static str>
  (None in the case of arrays and tuple structs).
• [breaking] Derive macro attributes: accessor/validation revamp: getter -> get, setter -> get_mut,
  and validate with more idiomatic and flexible usage and call sequencing.
• [breaking] Metadata.separator() has been changed to only return the new maximum length for
  consistency and renamed to max_length(separator: &str).

Added

• TreeAny to access nodes trough Any trait objects.
• TreeKey::json_path() for JSON path notation .bar[5]
• JsonPath: Keys
• rename field attribute for derive macros
• Counted iteration is supported (includung ExactSizeIterator) through the count()
  "augmentation" methods on the iterators.

Removed

• [breaking] TreeKey::iter_*_unchecked() have been removed. Uncounted iteration is the default.

[ryan-summers]

There's a lot here, so I went over it the best I could, but wasn't able to look at some of the implementation in detail.