The type that is returned when loading any taxonomies through that library.
It include 2 implementations of the Taxonomy trait: one using strings as ids
(easier to use but slower) and one using internal indices (harder to use but faster).
Most fields are public for flexibility reason.
The trait to implement for each form we implement.
It does include some default implementations for a few functions as well as a way
to iterate through it, starting with the specified node.