Add new signature introspection and cloning functions.
Review Request #9603 - Created Feb. 8, 2018 and updated
This change introduces new functions for the new signature classes,
providing helpers for fetching child signatures, defaults and values for
fields, and to clone whole signature trees. These will be used in
upcoming diffing and mutation changes.
There's also some normalization for the
index_togetherfields, ensuring we're working with data in a
Unit tests pass for all Django versions and for Python 2 and 3.