load_store and dump_to_store

[keewis]

Continuing from #3602, load_store and dump_to_store look like they are old and unmaintained functions:

• load_store is referenced once in api.rst (I assume the reference to from_store was to load_store), but never tested, used or mentioned anywhere else
• dump_to_store is tested (and probably used), but never mentioned except from the section on backends

what should we do with these? Are they obsolete and should be removed or just unmaintained (then we should properly document and test them).

[shoyer]

I would put these in the "obsolete and should be removed" category. These APIs need to be rationalized with the new modular backends API.