Revert "box: zap key_part_def struct"

This reverts commit ea3a2b5f.

Once we finally implement json path indexes, more fields that are
calculated at run time will have to be added to struct key_part, like
path hash or field offset. So this was actually a mistake to remove
key_part_def struct, as it will grow more and more different from
key_part. Conceptually having separate key_part_def and key_part is
consistent with other structures, e.g. tuple_field and field_def.
That said, let's bring key_part_def back. Sorry for the noise.