auto merge of #6434 : alexcrichton/rust/less-implicit-vecs, r=bstrie

This closes #5204 and #6421.

This also removes the `vecs_implicitly_copyable` lint (although now reading #6421, this may not be desired?). If we want to leave it in, it at least removes it from the compiler.
Get unit tests for rusti working

* They didn't work before, because the location of the tests caused the
  'sysroot' option to crate lookup to be wrong for finding the correct stage's
  core/std libraries. This moves the compiled tests from the $host/test
  directory into a $host/$stage/test directory. This means that the sysroot will
  be correct and the core/std libraries can actually be found
* The LLVM bindings apparently aren't threadsafe, so we can't run multiple tests
  in parallel.
Fix `use` statements with rusti
Add regression tests for various other rusti issues
Don't emit common warnings in rusti
Fix rustpkg tests now that they're in a different location