libcollection: generalize StrVector to AsSlice<Str>.

The impl for [T] also works as impl for slices in general.
By generalizing the impl of StrVector for Vec<Str> to that for
AsSlice<Str>, it becomes much more generic.

Once Iterable is implemented, we will prefer it to AsSlice.
But the with_capacity() part might become tricky.

Signed-off-by: NODA, Kai <nodakai@gmail.com>
auto merge of #19192 : nodakai/rust/generalize-strvector, r=alexcrichton

A single impl supports all of `[T]`, `Vec<T>` and `CVec<T>`.

Once `Iterable` is implemented, we will prefer it to `SlicePrelude`.
But the `with_capacity()` part might become tricky.