auto merge of #7605 : thestinger/rust/vec, r=Aatch

This is work continued from the now landed #7495 and #7521 pulls.

Removing the headers from unique vectors is another project, so I've separated the allocator.

diff --cc src/librustc/middle/trans/_match.rs
Simple merge

diff --cc src/librustc/middle/trans/common.rs
Simple merge

diff --cc src/librustc/middle/trans/expr.rs
Simple merge

diff --cc src/libstd/vec.rs
Simple merge