std,serialize: remove some internal uses of ~[].

These are all private uses of ~[], so can easily & non-controversially
be replaced with Vec.
std,native,green,rustuv: make readdir return `Vec`.

Replacing `~[]`. This also makes the `walk_dir` iterator use a `Vec`
internally.
std,syntax: make std::fmt::parse use `Vec`s.
green: de-~[].
native: remove some internal ~[].
Remove some internal ~[] from several libraries.

Some straggling instances of `~[]` across a few different libs. Also,
remove some public ones from workcache.
auto merge of #13425 : huonw/rust/general-devecing, r=brson

Remove a pile of (mainly) internal `~[]` uses

A few of them are public like the changes in `std::fmt::parse`, but the majority are just implementation details.