libstd: Add some methods to `Vec<T>`.
test: Automatically remove all `~[T]` from tests.
test: Make manual changes to deal with the fallout from removal of
`~[T]` in test, libgetopts, compiletest, librustdoc, and libnum.
auto merge of #12837 : pcwalton/rust/devecing-tests, r=brson

r? @brson