xml: use pointers rather than values in definitions of elements.
xml: use pointers rather than values in definitions of elements.

The rationale is that for the purposes of the API, we need to
record pointers to various elements however these pointers
will become stale if a slice is resized for some reason. By
storing slices of pointers instead, the pointers will remain
valid even if the slice is resized.

Signed-off-by: Jon Seymour <44f878afe53efc66b76772bd845eb65944ed8232@wildducktheories.com>

eightyeight/gosunspec,crabmusket/gosunspec