librustc: Remove unique vector patterns from the language.

Preparatory work for removing unique vectors from the language, which is
itself preparatory work for dynamically sized types.
auto merge of #12244 : pcwalton/rust/deuniquevectorpatterns, r=pcwalton

Preparatory work for removing unique vectors from the language, which is
itself preparatory work for dynamically sized types.

r? @brson
move extra::test to libtest