Implement archive size API (metaArchiveSize).

I don't think this is checking what we want it to. the assertCheckData functions are
for sanity checking the get function. By getting the data from the store, we're
sanity checking something against itself - that's why we don't pass the
store to the sanity checking functions.

I think we should lose the newly added *charmstore.Store argument and just
do:

    func sizeChecker(c *gc.C, data interface{}) {
       c.Assert(data.(int64), gc.Not(gc.Equals), int64(0))
    }

If we're getting something that's the right type, and it's not zero,
then I'm reasonably happy that the entityFieldGetter("Size")
call is doing the right thing.