This hook replaces lib.TranslateKeys and has a number of advantages:
1. Primarily, aliases for fields are defined on the field itself, making
the aliases much easier to maintain, and more obvious to the reader.
2. TranslateKeys translation rules are not aware of structure. It could
very easily incorrectly translate a key on one struct that was intended
to be a translation rule for a completely different struct, leading
to very hard to debug errors. The hook removes the need for the
unexpected "translation rule is an empty string to indicate stop
traversal" special case.
3. TranslateKeys attempts to duplicate a bunch of tree traversal logic
that already exists in mapstructure. Using mapstructure for traversal
removes the need to traverse the entire structure multiple times, and
makes the behaviour more obvious to the reader.
This change is being made to enable a future change of replacing
PatchSliceOfMaps. TranslateKeys sits in between PatchSliceOfMaps and
mapstructure.Decode, so it must be converted to a hook first, before
PatchSliceOfMaps can be replaced by a decode hook.
