Purge @-boxes from the reading half of EBML

Now that the metadata is an owned value with a lifetime of a borrowed byte
slice, it's possible to have future optimizations where the metadata doesn't
need to be copied around (very expensive operation).
auto merge of #11057 : alexcrichton/rust/no-at-in-ebml, r=pcwalton

Now that the metadata is an owned value with a lifetime of a borrowed byte
slice, it's possible to have future optimizations where the metadata doesn't
need to be copied around (very expensive operation).