fix decoding of multiple attributes (fixes #7017)

We were just looking at the last because we were overwriting ret.
add test for RcMut being Const and Owned, tests #7017 being fixed
auto merge of #7018 : bill-myers/rust/fix_7017, r=Aatch

Minimally fixes #7017, we were overwriting the result and thus ignoring attributes before the last.

csearch::get_item_attrs and decoder::get_item_attrs should probably also be changed to each_item_attrs using the for protocol, but that's just a minor performance/style issue.