hdf5: mark types with pointers
Created by: kortschak
This is part of the marking needed to know whether data copying will be needed. Note that it currently is overly conservative, and for all practical uses it will mark everything. There is a TODO that has an approach that I think will fix that.
This is more a PR for discussion of approach, but may end up being something that we can merge.
The half of the story needed to handle a Datatype
that has been returned from opening a file is not dealt with here.
Please take a look.
Updates #12.