Fix take glue of owned trait objects

This finishes the incomplete conversion of unique traits as two-word
allocations started in 211d038abc05c77785f72a31840016517cf218c2.

Fixes #5882, #6717, #7153, #7208.
auto merge of #7214 : Blei/rust/fix-owned-traits, r=pcwalton

This finishes the incomplete conversion of unique traits as two-word
allocations started in 211d038abc05c77785f72a31840016517cf218c2.

Fixes #5882, #6717, #7153, #7208.